Interconnecting Cisco Networking Devices – Part 1
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BY : S QAISAR SHAH
How the Transport Layer Works
The Transport layer- is the layer responsible for actually getting
the data packets to a specific location. When we receive email, we want to open
it with our email program- not anything else. So how does a computer know
exactly where to route data to appropriate programs, all while dealing with
multiple connections?
The Difference between TCP and UDP
There are two protocols that are primarily used to transport data:
TCP and UDP. TCP stands for transmission control protocol. It is the more
common of the two, since it allows for much more error checking functionality
and stability. UDP, or User Datagram Protocol, lacks extensive error checking-
but is considered to be much faster than TCP as a result.
Since TCP guarantees the delivery of data over a network we call it a connection-oriented protocol. If in the event that data isn’t sent correctly, the sending computer will be notified and will resend the information. This is compared to UDP, which doesn’t require that data has been received correctly. Likewise, we call UDP a connectionless protocol.
How Do Transport Protocols Work?
How Do Transport Protocols Work?
We mentioned earlier that a transport protocol can have simultaneous connections to a computer- yet the receiving computer still knows where each data packet should be sent. This is accomplished through ports and sockets.
A port is simply an internal address that acts as a pathway to control data flow. Since we need each port to be specific to a certain application, there are thousands of ports for use. If you are using the internet, for instance, data is being routed through TCP port 80. This port is called the HTTP port. Ports that have specific purposes (such as the HTTP port) are also known as well-known ports. (And in case you were wondering, there are over 65,535 ports to experiment with.
So now we know two things. First, the IP address is used to route the data to a specific computer. Next, the port number is used to tell the receiving computer what kind of application should handle it. But to actually accomplish both of these tasks, we use what is called a socket. A socket is simply an address formed by using the IP address and then tacking on the port number at the end.
Practical Uses of the Socket
2. Destination Port - A 16-bit field that specifies which port number the data segment is destined for on the receiving machine.
3. Sequence Number - A 32-bit field that specifies which sequence number the particular segment of information is assigned. The sequence number is used to number packets of information so that they may be counted on the receiving side- guaranteeing a successful and complete delivery of information.
4. Acknowledgment Number - A 32 bit field that specifies whether or not a segment was received correctly. The acknowledgment number is always one higher than the sequence number, since the receiving computer is expecting the next segment.
5. Data Offset – A 4-bit field that tells the receiving computer how long the header is, and where the data actually begins.
6. Reserved - A 6-bit field that is reserved for future use. Currently this field is represented as all zeroes. In the future, it may be likely that TCP will make use of this space for some reason or another.
7. URG - A 1-bit control flag that stands for urgent. If the value is 1, the information is urgent and should be dealt with accordingly.
8. ACK - A 1-bit control flag that, if set to 1, indicates that the Acknowledgment Number field is significant.
9. PSH - A 1-bit control flag that stands for push. If set to 1, all the information sent so far is sent to the receiving application.
10. RST - 1-bit control flag that stands for reset. If set to 1, the connection is reset.
11. SYN - A 1-bit control flag that stands for synchronize. If set to 1, then a sequence of numbers will be used to sort information packets. This also marks the beginning of a connection.
12. FIN - A 1-bit control flag that stands for finished. It also closes a connection, and indicates that there is no more data to be sent.
13. Window - A 16-bit field that is used for flow control. It indicates that a range of sequence numbers past the last acknowledged sequence number do not require further acknowledgment.
14. Checksum - A 16-bit field that checks segment integrity. A calculation is done on both the sending and receiving computer. This calculation is based on the segment’s information, so we can use it to check and see if the packet is indeed the same being received as it was sent.
15. Urgent Pointer - A 16-bit field that indicates the beginning of urgent information. Specifically, it points to a sequence number.
16. Options - A field that may be used to set various optional settings.
17. Padding - A spacer used to offset the Options field. Since every row must equal 32 bits, the Padding field must add to the Options field to equal 32 bits. Since the Options field may vary, variable Padding is needed.
18. Data - The actual data being sent to the recipient computer.
1. Source Port - An optional 16-bit field that specifies which port the datagram originated from.
2. Destination Port - A 16-bit field that specifies which port the datagram should be directed to on the receiving computer.
3. Length - A 16-bit field that specifies how long the UDP datagram is. This includes the UDP header and the data being sent. (The value is always at least 8, since the UDP header is eight octets.)
4. Checksum - A 16-bit field that operates much like the TCP counterpart. It is considered optional, however.
5. Data - The actual data being sent to the recipient computer.
You will notice that the datagram anatomy is much simpler- and that no source or destination information is included. So how does the data know where to go? As we briefly reviewed earlier, UDP uses what is known as a pseudo-header. This header will act as a guide for the datagram, and can determine whether the datagram was delivered to the right address or not.
So what good is a socket? For one thing, multiplexing and demultiplexing is made possible. When you are running multiple programs that are communicating with other computers, you are making use of both of these technologies.
Let’s look at a practical example. You are running a telnet server in which multiple computers are connected to. Each computer uses a socket address to tell the server which computer and which port the data is coming from. If each computer broadcasted at the same time, there may be a jam at the transport layer. We use multiplexing in this case to combine all incoming data into one stream.
Demultiplexing is very similar to multiplexing, except that it works in reverse. Instead of taking multiple streams of input data and outputting a single stream of data, demultiplexing involves receiving a single stream of input and delivering it to multiple outputs. This is handy for separating multiplexed signals. You may see demultiplexing referred to as “Demuxing.”
The Anatomy of a TCP Segment
So now we know how data is routed, but what is the data made up of? As we learned in previous sections, we call the data at the Transport level of the TCP protocol a segment.
The Anatomy of a TCP Segment
So now we know how data is routed, but what is the data made up of? As we learned in previous sections, we call the data at the Transport level of the TCP protocol a segment.
TCP Segments Explained
1. Source Port - A 16-bit field that specifies which port number the data segment originated from on the source machine.
1. Source Port - A 16-bit field that specifies which port number the data segment originated from on the source machine.
2. Destination Port - A 16-bit field that specifies which port number the data segment is destined for on the receiving machine.
3. Sequence Number - A 32-bit field that specifies which sequence number the particular segment of information is assigned. The sequence number is used to number packets of information so that they may be counted on the receiving side- guaranteeing a successful and complete delivery of information.
4. Acknowledgment Number - A 32 bit field that specifies whether or not a segment was received correctly. The acknowledgment number is always one higher than the sequence number, since the receiving computer is expecting the next segment.
5. Data Offset – A 4-bit field that tells the receiving computer how long the header is, and where the data actually begins.
6. Reserved - A 6-bit field that is reserved for future use. Currently this field is represented as all zeroes. In the future, it may be likely that TCP will make use of this space for some reason or another.
7. URG - A 1-bit control flag that stands for urgent. If the value is 1, the information is urgent and should be dealt with accordingly.
8. ACK - A 1-bit control flag that, if set to 1, indicates that the Acknowledgment Number field is significant.
9. PSH - A 1-bit control flag that stands for push. If set to 1, all the information sent so far is sent to the receiving application.
10. RST - 1-bit control flag that stands for reset. If set to 1, the connection is reset.
11. SYN - A 1-bit control flag that stands for synchronize. If set to 1, then a sequence of numbers will be used to sort information packets. This also marks the beginning of a connection.
12. FIN - A 1-bit control flag that stands for finished. It also closes a connection, and indicates that there is no more data to be sent.
13. Window - A 16-bit field that is used for flow control. It indicates that a range of sequence numbers past the last acknowledged sequence number do not require further acknowledgment.
14. Checksum - A 16-bit field that checks segment integrity. A calculation is done on both the sending and receiving computer. This calculation is based on the segment’s information, so we can use it to check and see if the packet is indeed the same being received as it was sent.
15. Urgent Pointer - A 16-bit field that indicates the beginning of urgent information. Specifically, it points to a sequence number.
16. Options - A field that may be used to set various optional settings.
17. Padding - A spacer used to offset the Options field. Since every row must equal 32 bits, the Padding field must add to the Options field to equal 32 bits. Since the Options field may vary, variable Padding is needed.
18. Data - The actual data being sent to the recipient computer.
The Anatomy of a UDP Datagram
Don’t worry- UDP is a lot simpler as compared to the TCP counterpart. This is due to the lack of error checking UDP contains. Although UDP is famously known for not having error checking, there are slight UDP functionalities that can act as basic error checking procedures.
It’s important to know that the UDP error checking functionalities are quite basic. For instance, the UDP protocol contains functionality for a checksum so that we may check the integrity of data. This field can be turned off, however, so that a faster connection can be had. Also important to note is that UDP uses a fake header (known as a pseudo-header) that contains a destination address, so that we can determine whether or not the packets were sent to the right place. If they are indeed sent to the wrong place, a simple ICMP message may notify the source machine that the source is unreachable.
Keep in mind that even if errors are found within a UDP connection, data will not be retransmitted. So how much different is a UDP datagram from a TCP segment?
UDP Datagrams Explained
1. Source Port - An optional 16-bit field that specifies which port the datagram originated from.
2. Destination Port - A 16-bit field that specifies which port the datagram should be directed to on the receiving computer.
3. Length - A 16-bit field that specifies how long the UDP datagram is. This includes the UDP header and the data being sent. (The value is always at least 8, since the UDP header is eight octets.)
4. Checksum - A 16-bit field that operates much like the TCP counterpart. It is considered optional, however.
5. Data - The actual data being sent to the recipient computer.
You will notice that the datagram anatomy is much simpler- and that no source or destination information is included. So how does the data know where to go? As we briefly reviewed earlier, UDP uses what is known as a pseudo-header. This header will act as a guide for the datagram, and can determine whether the datagram was delivered to the right address or not.
Subnet Chart IPv4
This is
an Internet Protocol (IPv4) Subnet Chart. You can use this to quickly look up
how you might need to subnet your network. At the bottom there is a quick
how-to on calculating subnets.
Class address ranges:
Class A = 1.0.0.0 to 126.0.0.0
Class B = 128.0.0.0 to 191.255.0.0
Class C = 192.0.1.0 to 223.255.255.0
Reserved address ranges for private (non-routed) .
10.0.0.0 -> 10.255.255.255
172.16.0.0 -> 172.31.255.255
192.168.0.0 -> 192.168.255.255
Other reserved addresses:
127.0.0.0 is reserved for loopback and IPC on the local host
224.0.0.0 -> 239.255.255.255 is reserved for multicast addresses
Chart notes:
Number of Subnets - "( )" Refers to the number of effective subnets, since the use of subnet numbers of all 0s or all 1s is highly frowned upon and RFC non-compliant.
Number of Hosts - Refers to the number of effective hosts, excluding the network and broadcast address.
Class A
Class address ranges:
Class A = 1.0.0.0 to 126.0.0.0
Class B = 128.0.0.0 to 191.255.0.0
Class C = 192.0.1.0 to 223.255.255.0
Reserved address ranges for private (non-routed) .
10.0.0.0 -> 10.255.255.255
172.16.0.0 -> 172.31.255.255
192.168.0.0 -> 192.168.255.255
Other reserved addresses:
127.0.0.0 is reserved for loopback and IPC on the local host
224.0.0.0 -> 239.255.255.255 is reserved for multicast addresses
Chart notes:
Number of Subnets - "( )" Refers to the number of effective subnets, since the use of subnet numbers of all 0s or all 1s is highly frowned upon and RFC non-compliant.
Number of Hosts - Refers to the number of effective hosts, excluding the network and broadcast address.
Class A
Network Bits
|
Subnet Mask
|
Number of Subnets
|
Number of Hosts
|
/8
|
255.0.0.0
|
0
|
16777214
|
/9
|
255.128.0.0
|
2 (0)
|
8388606
|
/10
|
255.192.0.0
|
4 (2)
|
4194302
|
/11
|
255.224.0.0
|
8 (6)
|
2097150
|
/12
|
255.240.0.0
|
16 (14)
|
1048574
|
/13
|
255.248.0.0
|
32 (30)
|
524286
|
/14
|
255.252.0.0
|
64 (62)
|
262142
|
/15
|
255.254.0.0
|
128 (126)
|
131070
|
/16
|
255.255.0.0
|
256 (254)
|
65534
|
/17
|
255.255.128.0
|
512 (510)
|
32766
|
/18
|
255.255.192.0
|
1024 (1022)
|
16382
|
/19
|
255.255.224.0
|
2048 (2046)
|
8190
|
/20
|
255.255.240.0
|
4096 (4094)
|
4094
|
/21
|
255.255.248.0
|
8192 (8190)
|
2046
|
/22
|
255.255.252.0
|
16384 (16382)
|
1022
|
/23
|
255.255.254.0
|
32768 (32766)
|
510
|
/24
|
255.255.255.0
|
65536 (65534)
|
254
|
/25
|
255.255.255.128
|
131072 (131070)
|
126
|
/26
|
255.255.255.192
|
262144 (262142)
|
62
|
/27
|
255.255.255.224
|
524288 (524286)
|
30
|
/28
|
255.255.255.240
|
1048576 (1048574)
|
14
|
/29
|
255.255.255.248
|
2097152 (2097150)
|
6
|
/30
|
255.255.255.252
|
4194304 (4194302)
|
2
|
Class B
Network Bits
|
Subnet
Mask
|
Number
of Subnets
|
Number
of Hosts
|
/16
|
255.255.0.0
|
0
|
65534
|
/17
|
255.255.128.0
|
2 (0)
|
32766
|
/18
|
255.255.192.0
|
4 (2)
|
16382
|
/19
|
255.255.224.0
|
8 (6)
|
8190
|
/20
|
255.255.240.0
|
16 (14)
|
4094
|
/21
|
255.255.248.0
|
32 (30)
|
2046
|
/22
|
255.255.252.0
|
64 (62)
|
1022
|
/23
|
255.255.254.0
|
128 (126)
|
510
|
/24
|
255.255.255.0
|
256 (254)
|
254
|
/25
|
255.255.255.128
|
512 (510)
|
126
|
/26
|
255.255.255.192
|
1024 (1022)
|
62
|
/27
|
255.255.255.224
|
2048 (2046)
|
30
|
/28
|
255.255.255.240
|
4096 (4094)
|
14
|
/29
|
255.255.255.248
|
8192 (8190)
|
6
|
/30
|
255.255.255.252
|
16384 (16382)
|
2
|
Class C
Network
Bits
|
Subnet
Mask
|
Number
of Subnets
|
Number
of Hosts
|
/24
|
255.255.255.0
|
0
|
254
|
/25
|
255.255.255.128
|
2 (0)
|
126
|
/26
|
255.255.255.192
|
4 (2)
|
62
|
/27
|
255.255.255.224
|
8 (6)
|
30
|
/28
|
255.255.255.240
|
16 (14)
|
14
|
/29
|
255.255.255.248
|
32 (30)
|
6
|
/30
|
255.255.255.252
|
64 (62)
|
2
|
Supernetting (CIDR) Chart
CIDR - Classless Inter-Domain Routing.
Note: The Number of Class C networks must be contiguous.
For example, 192.169.1.0/22 represents the following block of addresses:
192.169.1.0, 192.169.2.0, 192.169.3.0 and 192.169.4.0.
Class C
CIDR - Classless Inter-Domain Routing.
Note: The Number of Class C networks must be contiguous.
For example, 192.169.1.0/22 represents the following block of addresses:
192.169.1.0, 192.169.2.0, 192.169.3.0 and 192.169.4.0.
Class C
CIDR Block
|
Supernet Mask
|
Number of Class C Addresses
|
Number of Hosts
|
/14
|
255.252.0.0
|
1024
|
262144
|
/15
|
255.254.0.0
|
512
|
131072
|
/16
|
255.255.0.0
|
256
|
65536
|
/17
|
255.255.128.0
|
128
|
32768
|
/18
|
255.255.192.0
|
64
|
16384
|
/19
|
255.255.224.0
|
32
|
8192
|
/20
|
255.255.240.0
|
16
|
4096
|
/21
|
255.255.248.0
|
8
|
2048
|
/22
|
255.255.252.0
|
4
|
1024
|
/23
|
255.255.254.0
|
2
|
512
|
Quick Subnetting How-To
[Understanding decimal - Base 10]
The first thing you must know is that the common number system used world wide is thedecimal system (otherwise known as base 10). What makes the decimal system a base 10 system is that it is based on grouping numbers by 10's. It is believed that the system evolved because we have ten fingers and ten toes which over the years we have used for counting. I use mine all the time (grin). We name the ten digits: zero, one, two, three, four, five, six, seven, eight and nine.
The decimal system has a 1's place, a 10's place, a 100's place, a 1000's place and so on. We say the number places are grouped by 10's because multiplying each number place by 10 gives you the next number place. So: 1x10=10 (the 10's place), 10x10=100 (the 100's place), 100x10=1000 (the 1000's place) etc.
Let's look at the decimal number 103 by place.
103 <- read from right to left
We have a 3 in the 1's place
We have a 0in the 10's place
We have a 1 in the 100's place
Thus: 100+0+3=103
By now you probably feel like you have attended Kindergarten for the second time in your life? Sorry about that but it is very important that you understand the concept of what a number system is, and what it is based on before we look at binary.
[Understanding binary - base 2]
Binary is a base 2 system, and thus groups numbers by 2's and not by 10's like the decimal system. We name the two digits: zero and one. The binary system has a 1's place, a 2's place, a4's place, an 8's place, a 16's place and so on. We say the number places are grouped by 2's because multiplying each number place by 2 gives you the next number place. So: 1x2=2 (the 2's place), 2x2=4 (the 4's place), 4x2=8 (the 8's place), 8x2=16 (the 16's place) etc.
Let's look at the decimal number Let's look at the decimal number 103 in binary format:
01100111 <- read from right to left
We have a 1 in the 1's place
We have a 1 in the 2's place
We have a 1 in the 4's place
We have a 0 in the 8's place
We have a 0 in the 16's place
We have a 1 in the 32's place
We have a 1 in the 64's place
We have a 0 in the 128's place
Thus: 0+64+32+0+0+4+2+1=103
Okay, Let's test your skills. Here is a list of binary numbers, try converting them to decimal and check your answers at the end of this post.
10000000
11000000
11100000
01000000
10000011
10010001
11111111
If you were able to convert these numbers to decimal then congratulations! You're ready to move on to the next section.
[Understanding a subnet mask]
Now that you understand what binary is, let's have a look at our two subnet masks from the beginning of my post:
192.168.1.0 / 255.255.255.0
192.168.1.0/24
The concept of a subnet mask is simple. You have a network and you have hosts on the network (anything with an IP address is a host). The subnet mask determines what portion of the TCP/IP address represents your network and what portion can be used for your hosts. Because I am a simple person, I think of it like this; The network number represents the street I live on, and the host portion is used for the numbers on all the houses on my street.
A subnet mask of 255.255.255.0 means that the first three octets of the address will be used for the network, and thus our network number is 192.168.1. This means we can have 254computers on this network, because the fourth octet is not being used by the network portion of the address. We know this because of the 0 in the subnet mask (255.255.255.0).
We call each of the number sections an octet because we think of them in binary, and there are eight possible bits in each section. Eight bits is an octet. 11111111 in binary is 255 in decimal (did you do the conversions?). So our decimal subnet mask 255.255.255.0 displayed in binary is going to be:
11111111.11111111.11111111.00000000
If you count all the ones, you will find that there are 24 of them. Now look at the subnet mask examples again.
192.168.1.0/255.255.255.0
192.168.1.0/24
Do you see why both subnet masks are the same? The number 24 is the number of bits used in the network portion of the address, and is short-hand for writing the address/subnet mask combination. It becomes important to understand this when you start dividing your network into multiple sub networks.
[Understanding Subnetting]
Before reading this section, you should have a good understanding of what a subnet mask is and how binary bits represent the subnet mask.
Simply put, subnetting is dividing your network into multiple sub networks. To go back to my silly example about houses and streets, subnetting gives you multiple streets in your neighborhood.
There are two methods for dividing your network into multiple sub networks; One is to simply change your network numbers keeping the same subnet mask. The other is to subnet your network into smaller sub networks.
Keeping the same mask:
Your network could be divided into two or more networks by changing the network portion of the address such as 192.168.1 and 192.168.2 and keeping the same subnet mask.
Example:
192.168.1.0/255.255.255.0
192.168.2.0/255.255.255.0
Doing this would give you two separate networks with 254 hosts per network. This is a very common method of dealing with multiple networks. However, back in the good old days you had to pay for every IP address you used, and if you had 25 computers on your network you probably would not want to pay for 254 addresses! The answer to the problem is...subnetting.
Subnetting a network:
Subnetting is when you use bits from the host portion of your address as part of your network number. This let's you subdivide your network at the cost of host addresses, which is great if you're paying for every host IP address. It will save you money because you pay for fewer TCP/IP addresses. Confused? Here is where understanding binary is important.
Lets look at a new subnet mask:
255.255.255.224
As you can see in the fourth octet, some of the host portion of this subnet mask is now being used for part of the network address. Which means we are now using some of the binary bits in the fourth octet for our network numbers, and that gives us fewer hosts than our old mask (which gave us 254), but gives us more networks (which is why we call it subnetting).
How can we tell how many networks and hosts per network this new subnet mask will give us? Well... we shall have to use some of our newly acquired binary skills.
The first task is to find out how many bits in the fourth octet are being used? The decimal number is 224, what is the decimal number 224 as represented in binary?
The decimal number 224 in binary is:
11100000
We have a 0 in the 1's place
We have a 0 in the 2's place
We have a 0 in the 4's place
We have a 0 in the 8's place
We have a 0 in the 16's place
We have a 1 in the 32's place
We have a 1 in the 64's place
We have a 1 in the 128's place
Thus: 128+64+32+0+0+0+0+0=224
So our complete subnet mask in binary is:
1111111.11111111.11111111.11100000
We now know that three bits from the fourth octet are used. How can we tell how many sub networks we're going to have? This requires some math- sorry. The formula is: 2n-2, where n is the number of bits being used from the host portion of our subnet mask.
Note: We subtract 2 from the total because you do not count all 0's or all 1's.
The formula for three bits is:
23-2=6
In simpler terms:
(2x2x2)-2=6
So our network is sub divided into 6 networks. Next, we want to know what the network numbers are, and how many hosts we can have on each of the 6 networks?
What is the first subnet? Let's have a look at the bits in our fourth octet again. The bit that gives us the answer is the (1) closest to the first zero, and in this case it is the 3rd bit from the left.
11100000
The 3rd bit will start our first network, and the 3rd bit is in the 32's place (remember binary). Start adding the value 32 to itself six times to get the six network numbers.
Note: A quicker way to find our starting network number is to subtract our mask from 256.
256-224=32
Here are our network numbers:
32
64
96
128
160
192
A better way to display this is:
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
The host addresses will fall between the network numbers, so we will have 30 hosts per network. You're probably wondering why it's not 31? The answer is that the last address of each subnet is used as the broadcast address for that subnet.
Example:
Subnet:192.168.1.32 / 255.255.255.224
Address Range: 192.168.1.33 through 192.168.1.62 (30 hosts)
Subnet Broadcast Address:192.168.1.63
Quiz:
Let's test your skills- write the address range and broadcast address for the following subnet. You will find the answer at the end of this post.
Subnet: 192.168.1.128 / 255.255.255.224
Address Range?
Subnet Broadcast Address?
If we we're paying for our TCP/IP addresses, we would only pay for one network and host combination, thus paying for 30 hosts and not 254. It could mean some real savings, it also frees up the remaining addresses for other organizations to use.
Let's look at another subnet mask:
255.255.255.240
How many bits are used from the host portion? To find this out, we need to know how the decimal number 240 is represented in binary.
The answer is:
11110000
So four bits are taken from the host portion of our mask. We do the same math as before:
24-2=14
In simpler terms:
(2x2x2x2)-2=14
We will have 14 sub networks, and what will the network numbers be? Look at the fourth bit, it's in the 16's place:
11110000
Note: A quicker way to find our starting network number is to subtract the value of our mask from 256. So: 256-240=16
Start adding 16 to itself- fourteen times to get all 14 network numbers:
16
32
48
64
80
96
112
128
144
160
176
192
208
224
A better way to display our subnets is:
192.168.1.16
192.168.1.32
192.168.1.48
192.168.1.64
192.168.1.80
192.168.1.96
192.168.1.112
192.168.1.128
192.168.1.144
192.168.1.160
192.168.1.176
192.168.1.192
192.168.1.208
192.168.1.224
The host addresses fall between the network numbers. So we will have 14 host addresses on each of our 14 sub networks (remember: the last or 15th address is the broadcast address for that subnet).
If you had a small company with 10 hosts and needed to have a static IP address for all of your hosts, you would be assigned a network/subnet mask and a valid IP address range.
Here is an example of what that might look like:
Network: 205.112.10.16/.255.255.255.240
Address Range: 205.112.10.17 through 205.112.10.30
Subnet Broadcast Address: 205.112.10.31
[Answers to Binary Conversions]
10000000 = 128
11000000 = 192
11100000 = 224
01000000 = 64
10000011 = 131
10010001 = 145
11111111 = 255
[Answer to Subnet Question]
Subnet:192.168.1.128 / 255.255.255.224
Address Range: 192.168.1.129 through 192.168.1.158
Subnet Broadcast Address: 192.168.1.159
The first thing you must know is that the common number system used world wide is thedecimal system (otherwise known as base 10). What makes the decimal system a base 10 system is that it is based on grouping numbers by 10's. It is believed that the system evolved because we have ten fingers and ten toes which over the years we have used for counting. I use mine all the time (grin). We name the ten digits: zero, one, two, three, four, five, six, seven, eight and nine.
The decimal system has a 1's place, a 10's place, a 100's place, a 1000's place and so on. We say the number places are grouped by 10's because multiplying each number place by 10 gives you the next number place. So: 1x10=10 (the 10's place), 10x10=100 (the 100's place), 100x10=1000 (the 1000's place) etc.
Let's look at the decimal number 103 by place.
103 <- read from right to left
We have a 3 in the 1's place
We have a 0in the 10's place
We have a 1 in the 100's place
Thus: 100+0+3=103
By now you probably feel like you have attended Kindergarten for the second time in your life? Sorry about that but it is very important that you understand the concept of what a number system is, and what it is based on before we look at binary.
[Understanding binary - base 2]
Binary is a base 2 system, and thus groups numbers by 2's and not by 10's like the decimal system. We name the two digits: zero and one. The binary system has a 1's place, a 2's place, a4's place, an 8's place, a 16's place and so on. We say the number places are grouped by 2's because multiplying each number place by 2 gives you the next number place. So: 1x2=2 (the 2's place), 2x2=4 (the 4's place), 4x2=8 (the 8's place), 8x2=16 (the 16's place) etc.
Let's look at the decimal number Let's look at the decimal number 103 in binary format:
01100111 <- read from right to left
We have a 1 in the 1's place
We have a 1 in the 2's place
We have a 1 in the 4's place
We have a 0 in the 8's place
We have a 0 in the 16's place
We have a 1 in the 32's place
We have a 1 in the 64's place
We have a 0 in the 128's place
Thus: 0+64+32+0+0+4+2+1=103
Okay, Let's test your skills. Here is a list of binary numbers, try converting them to decimal and check your answers at the end of this post.
10000000
11000000
11100000
01000000
10000011
10010001
11111111
If you were able to convert these numbers to decimal then congratulations! You're ready to move on to the next section.
[Understanding a subnet mask]
Now that you understand what binary is, let's have a look at our two subnet masks from the beginning of my post:
192.168.1.0 / 255.255.255.0
192.168.1.0/24
The concept of a subnet mask is simple. You have a network and you have hosts on the network (anything with an IP address is a host). The subnet mask determines what portion of the TCP/IP address represents your network and what portion can be used for your hosts. Because I am a simple person, I think of it like this; The network number represents the street I live on, and the host portion is used for the numbers on all the houses on my street.
A subnet mask of 255.255.255.0 means that the first three octets of the address will be used for the network, and thus our network number is 192.168.1. This means we can have 254computers on this network, because the fourth octet is not being used by the network portion of the address. We know this because of the 0 in the subnet mask (255.255.255.0).
We call each of the number sections an octet because we think of them in binary, and there are eight possible bits in each section. Eight bits is an octet. 11111111 in binary is 255 in decimal (did you do the conversions?). So our decimal subnet mask 255.255.255.0 displayed in binary is going to be:
11111111.11111111.11111111.00000000
If you count all the ones, you will find that there are 24 of them. Now look at the subnet mask examples again.
192.168.1.0/255.255.255.0
192.168.1.0/24
Do you see why both subnet masks are the same? The number 24 is the number of bits used in the network portion of the address, and is short-hand for writing the address/subnet mask combination. It becomes important to understand this when you start dividing your network into multiple sub networks.
[Understanding Subnetting]
Before reading this section, you should have a good understanding of what a subnet mask is and how binary bits represent the subnet mask.
Simply put, subnetting is dividing your network into multiple sub networks. To go back to my silly example about houses and streets, subnetting gives you multiple streets in your neighborhood.
There are two methods for dividing your network into multiple sub networks; One is to simply change your network numbers keeping the same subnet mask. The other is to subnet your network into smaller sub networks.
Keeping the same mask:
Your network could be divided into two or more networks by changing the network portion of the address such as 192.168.1 and 192.168.2 and keeping the same subnet mask.
Example:
192.168.1.0/255.255.255.0
192.168.2.0/255.255.255.0
Doing this would give you two separate networks with 254 hosts per network. This is a very common method of dealing with multiple networks. However, back in the good old days you had to pay for every IP address you used, and if you had 25 computers on your network you probably would not want to pay for 254 addresses! The answer to the problem is...subnetting.
Subnetting a network:
Subnetting is when you use bits from the host portion of your address as part of your network number. This let's you subdivide your network at the cost of host addresses, which is great if you're paying for every host IP address. It will save you money because you pay for fewer TCP/IP addresses. Confused? Here is where understanding binary is important.
Lets look at a new subnet mask:
255.255.255.224
As you can see in the fourth octet, some of the host portion of this subnet mask is now being used for part of the network address. Which means we are now using some of the binary bits in the fourth octet for our network numbers, and that gives us fewer hosts than our old mask (which gave us 254), but gives us more networks (which is why we call it subnetting).
How can we tell how many networks and hosts per network this new subnet mask will give us? Well... we shall have to use some of our newly acquired binary skills.
The first task is to find out how many bits in the fourth octet are being used? The decimal number is 224, what is the decimal number 224 as represented in binary?
The decimal number 224 in binary is:
11100000
We have a 0 in the 1's place
We have a 0 in the 2's place
We have a 0 in the 4's place
We have a 0 in the 8's place
We have a 0 in the 16's place
We have a 1 in the 32's place
We have a 1 in the 64's place
We have a 1 in the 128's place
Thus: 128+64+32+0+0+0+0+0=224
So our complete subnet mask in binary is:
1111111.11111111.11111111.11100000
We now know that three bits from the fourth octet are used. How can we tell how many sub networks we're going to have? This requires some math- sorry. The formula is: 2n-2, where n is the number of bits being used from the host portion of our subnet mask.
Note: We subtract 2 from the total because you do not count all 0's or all 1's.
The formula for three bits is:
23-2=6
In simpler terms:
(2x2x2)-2=6
So our network is sub divided into 6 networks. Next, we want to know what the network numbers are, and how many hosts we can have on each of the 6 networks?
What is the first subnet? Let's have a look at the bits in our fourth octet again. The bit that gives us the answer is the (1) closest to the first zero, and in this case it is the 3rd bit from the left.
11100000
The 3rd bit will start our first network, and the 3rd bit is in the 32's place (remember binary). Start adding the value 32 to itself six times to get the six network numbers.
Note: A quicker way to find our starting network number is to subtract our mask from 256.
256-224=32
Here are our network numbers:
32
64
96
128
160
192
A better way to display this is:
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
The host addresses will fall between the network numbers, so we will have 30 hosts per network. You're probably wondering why it's not 31? The answer is that the last address of each subnet is used as the broadcast address for that subnet.
Example:
Subnet:192.168.1.32 / 255.255.255.224
Address Range: 192.168.1.33 through 192.168.1.62 (30 hosts)
Subnet Broadcast Address:192.168.1.63
Quiz:
Let's test your skills- write the address range and broadcast address for the following subnet. You will find the answer at the end of this post.
Subnet: 192.168.1.128 / 255.255.255.224
Address Range?
Subnet Broadcast Address?
If we we're paying for our TCP/IP addresses, we would only pay for one network and host combination, thus paying for 30 hosts and not 254. It could mean some real savings, it also frees up the remaining addresses for other organizations to use.
Let's look at another subnet mask:
255.255.255.240
How many bits are used from the host portion? To find this out, we need to know how the decimal number 240 is represented in binary.
The answer is:
11110000
So four bits are taken from the host portion of our mask. We do the same math as before:
24-2=14
In simpler terms:
(2x2x2x2)-2=14
We will have 14 sub networks, and what will the network numbers be? Look at the fourth bit, it's in the 16's place:
11110000
Note: A quicker way to find our starting network number is to subtract the value of our mask from 256. So: 256-240=16
Start adding 16 to itself- fourteen times to get all 14 network numbers:
16
32
48
64
80
96
112
128
144
160
176
192
208
224
A better way to display our subnets is:
192.168.1.16
192.168.1.32
192.168.1.48
192.168.1.64
192.168.1.80
192.168.1.96
192.168.1.112
192.168.1.128
192.168.1.144
192.168.1.160
192.168.1.176
192.168.1.192
192.168.1.208
192.168.1.224
The host addresses fall between the network numbers. So we will have 14 host addresses on each of our 14 sub networks (remember: the last or 15th address is the broadcast address for that subnet).
If you had a small company with 10 hosts and needed to have a static IP address for all of your hosts, you would be assigned a network/subnet mask and a valid IP address range.
Here is an example of what that might look like:
Network: 205.112.10.16/.255.255.255.240
Address Range: 205.112.10.17 through 205.112.10.30
Subnet Broadcast Address: 205.112.10.31
[Answers to Binary Conversions]
10000000 = 128
11000000 = 192
11100000 = 224
01000000 = 64
10000011 = 131
10010001 = 145
11111111 = 255
[Answer to Subnet Question]
Subnet:192.168.1.128 / 255.255.255.224
Address Range: 192.168.1.129 through 192.168.1.158
Subnet Broadcast Address: 192.168.1.159
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