# WORKING OUT IPV4 LAN ADDRESS RANGES

## Introduction

There are occasions when you may need to work out the first and last IP addresses in a network given only an IP address in that network and its subnet mask. This is useful when calculating the number of addresses in the network or when you need to process each IP address separately.

## AN EXAMPLE

The following example shows how the calculation works. First, choose an IP address in the range and the subnet mask.Current network address 81.138.219.45 = 01010001.10001010.11011011.00101101 Subnet mask 255.255.255.248 = 11111111.11111111.11111111.11111000

To find the first address, which is also the network address, you bit-wise AND the given IP address and the subnet mask:

81.138.219.45 = 01010001.10001010.11011011.00101101 255.255.255.248 = 11111111.11111111.11111111.11111000 AND ----------------------------------------------------- 01010001.10001010.11011011.00101000 gives 81.138.219.40

To find the last address, which is also the broadcast address, you OR the given IP address with the one's complement of the subnet mask. The one's complement of the mask is obtained by flipping each bit.

Subnet mask 255.255.255.248 = 11111111.11111111.11111111.11111000 All bits flipped 0.0.0.7 = 00000000.00000000.00000000.00000111 81.138.219.45 = 01010001.10001010.11011011.00101101 Complement of mask 0.0.0.7 = 00000000.00000000.00000000.00000111 OR ----------------------------------------------------- 01010001.10001010.11011011.00101111 gives 81.138.219.47

Using this calculation you can determine:

- the first and last address in the range
- the network address - the first address
- the broadcast address - the last address (but also 255.255.255.255)
- the number of addresses in the range - simply the first subtracted from the last. You can also count the number of 0 bits in the mask and then calculate
two to this power. In this example there are three zeroes in the mask, so the number of addresses is 2
^{3}= 8. - how many "useful" addresses you have - usually the number in the network minus the network address, the broadcast address and the router address. In this example 2
^{3}- 3 = 5.

## FIXED IP ADDRESSES

Using the calculation above shows why, when you order a broadband service with fixed addresses, you can only have 1, 5 or 13 available addresses, etc. The 13 arises with a 28-bit subnet mask giving 16 addresses of which the network, router and broadcast each use one, leaving 13 for customer use.

## COMPLEMENTS

There are two types of bit complements:

- a one's complement that just switches or flips each bit. This is the operation that the C ~ operator performs.
- a two's complement that is a method of representing signed numbers in a computer. You convert between a positive integer and its negative counterpart by flipping all bits and adding one. The same process is used to reverse the calculation.