How are IP addresses and subnets chosen?

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Problem: How are IP addresses and subnets chosen?




Address:The unique number ID assigned to one host or interface in a network.

Subnet: A portion of a network, which may be a physically independent network segment, which shares a network address with other portions of the network and is distinguished by a subnet number.

Subnet mask: A 32-bit combination used to describe which portion of an address refers to the subnet and which part refers to the host.

Interface: A network connection.


Understanding IP Addresses

An IP address is an address used to uniquely identify a device on an IP network. The address is made up of 32 binary bits which can be divisible into two portions ith the help of a subnet mask: A network portion & a host portion.

The 32 binary bits are broken into four octets (1 octet = 8 bits).

Each octet is converted to decimal and separated by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example, The value in each octet ranges from 0 to 255 decimal, or 00000000 – 11111111 binary.

Here is how binary octets convert to decimal: The right most bit, or least significant bit, of an octet will hold a value of 20. The bit just to the left of that will hold a value of 21. This continues until the left-most bit, or most significant bit, which will hold a value of 27. So if all binary bits are a one, the decimal equivalent would be 255 as shown below.

  1 1 1 1 1 1 1 1
  128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=255)

Here is a sample octet conversion when not all of the bits are set to 1.

  0 1 0 0 0 0 0 1
  0 64 0 0 0 0 0 1 (0+64+0+0+0+0+0+1=65)

And this is sample shows an IP address represented in both binary and decimal.

  10. 1. 23. 19 (decimal)
  00001010.00000001.00010111.00010011 (binary)

These octets are broken down to provide an addressing scheme that can accommodate large and small networks.

There are five different classes of networks, A to E. This document focuses on addressing classes A to C, since classes D and E are reserved and discussion of them is beyond the scope of this document.

Note:Also note that the terms “Class A, Class B” and so on are used in this document to help facilitate the understanding of IP addressing and subnetting. These terms are rarely used in the industry anymore because of the introduction of classless intra domain routing (CIDR).


Network Mask

A network mask helps you know which portion of the address identifies the network and which portion of the address identifies the node. Class A, B, and C networks have default masks, also known as natural masks, as shown below.

Class A:

Class B:

Class C:

An IP address on a Class A network that has not been subnetted would have an address/mask pair similar to:


To see how the mask helps you identify the network and node parts of the address, convert the address and mask to binary numbers. = 00001000.00010100.00001111.00000001 = 11111111.00000000.00000000.00000000

Once you have the address and the mask represented in binary, then identifying the network and host ID is easier. Any address bits which have corresponding mask bits set to 1 represent the network ID. Any address bits that have corresponding mask bits set to 0 represent the node ID. = 00001000.00010100.00001111.00000001 = 11111111.00000000.00000000.00000000
 -------------net id-|-------host id------------
 netid = 00001000 = 8
 hostid = 00010100.00001111.00000001 = 20.15.1


Understanding Subnetting

Definition of Subnet A portion of a network, which may be a physically independent network segment, which shares a network address with other portions of the network and is distinguished by a subnet number.

Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network. If you do not subnet, you will only be able to use one network from your Class A, B, or C network, which is unrealistic.

Each data link on a network must have a unique network ID, with every node on that link being a member of the same network.

If you break a major network (Class A, B, or C) into smaller subnetworks, it allows you to create a network of interconnecting subnetworks.

Each data link on this network would then have a unique network/subnetwork ID. Any device, or gateway, connecting n networks/subnetworks has n distinct IP addresses, one for each network /subnetwork that it interconnects.

To subnet a network, extend the natural mask using some of the bits from the host ID portion of the address to create a subnetwork ID.

For example, given a Class C network of which has a natural mask of, you can create subnets in the following manner. - 11001100.00001111.00000101.00000000 - 11111111.11111111.11111111.11100000

By extending the mask to be, you have taken three bits (seen above as “sub”) from the original host portion of the address and used them to make subnets. With these three bits, it is possible to create eight subnets. With the remaining five host ID bits, each of the 8 subnets can have up to 32 host addresses, 30 of which can actually be assigned to a device since host ids of all zeros or all ones are not allowed (it is very important to remember this).

So, with this in mind, the following subnets have been created. host address range 1 to 30 host address range 33 to 62 host address range 65 to 94 host address range 96 to 126 host address range 129 to 158 host address range 161 to 190 host address range 193 to 222 host address range 225 to 254

Note: There are two ways to denote the above masks.

First, since you are using three bits more than the “natural” Class C mask, you can denote these addresses as having a 3-bit subnet mask.

Secondly, the mask of can also be denoted as /27 as there are 27 bits (24 from the 1st 3 octets and three from the host octet) that are set in the mask. This second method is used with CIDR. Using this method, one of the above networks can be described with the notation prefix/length. For example, denotes the network subnet

When appropriate the prefix/length notation is used.

The more host bits you use for a subnet mask, the more subnets you have available. However, the more subnets available, the less host addresses available per subnet. For example, a Class C network of and a mask of (/27) allows you to have eight subnets, each with 32 host addresses (30 of which could be assigned to devices). If you use a mask of (/28), the break down is as follows. - 11001100.00001111.00000101.00000000 - 11111111.11111111.11111111.11110000

Since you now have four bits to make subnets with, you only have four bits left for host addresses. So in this case you can have up to 16 subnets, each of which can have up to 16 host addresses (14 of which can be assigned to devices).

Take a look at how a Class B network might be subnetted. If you have network ,then you know that its natural mask is or Extending the mask to anything beyond means you are subnetting. You can quickly see that you have the ability to create a lot more subnets than with the Class C network above. If you use a mask of (/21), how many subnets and hosts per subnet does this allow for? - 10101100.00010000.00000000.00000000 - 11111111.11111111.11111000.00000000

You are using five bits from the original host bits for subnets. This will allow you to have 32 subnets (25). After using the five bits for subnetting, you are left with 11 bits for host addresses. This will allow each subnet so have 2048 host addresses (211), 2046 of which could be assigned to devices.

Note: In the past, there were limitations to the use of a subnet 0 (all subnet bits are set to zero) and all ones subnet (all subnet bits set to one). Some devices would not allow the use of these subnets.

Cisco Systems currently is a known network provider of equipment that will allow the use of these subnets when their ip subnet zero command is configured.

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