6 IPv4 Addressing and Subnetting

IPv4 addresses and prefixes

An IPv4 address is a 32-bit value, usually written as four decimal octets separated by periods. Each octet ranges from 0 to 255, as in 192.168.10.77.

A subnet mask separates the network prefix from the host portion of the address. In binary, the network bits are consecutive 1s, followed by 0s for the host bits. CIDR notation expresses the prefix length as a slash followed by the number of network bits. For example, /26 has 26 network bits and 6 host bits.

Common prefix-to-mask examples are:

• /24 corresponds to 255.255.255.0.

• /26 corresponds to 255.255.255.192.

• /27 corresponds to 255.255.255.224.

CIDR allows networks to use prefix lengths suited to their needs rather than relying on the old fixed address classes.

Network, broadcast, and host addresses

In an ordinary IPv4 subnet, the network address has every host bit set to 0, and the directed broadcast address has every host bit set to 1. Addresses between these endpoints are normally usable for hosts.

If the number of host bits is $h$, the total number of addresses and the usual number of usable host addresses are:

The subtraction excludes the network and broadcast addresses. There are exceptions: on point-to-point links, both addresses in a /31 can be used as endpoint addresses, while a /32 represents one individual address.

Find a subnet from an address

For 192.168.10.77/26, the prefix leaves 6 host bits. This gives 64 addresses per subnet and normally 62 usable host addresses. The mask is 255.255.255.192; the last-octet block size is $256 - 192 = 64$.

The subnet ranges in the last octet therefore begin at 0, 64, 128, and 192. Since 77 falls in the 64–127 block, the address information is:

• Network: 192.168.10.64

• First usable host: 192.168.10.65

• Last usable host: 192.168.10.126

• Broadcast: 192.168.10.127

Check that the given host address falls between the network and broadcast addresses.

Subnetting a network

To divide 192.168.10.0/24 into /27 subnets, the new prefix borrows 3 bits from the original /24. The number of resulting subnets is $2^3 = 8$. Each subnet has 5 host bits, so it has $2^5 - 2 = 30$ usable host addresses under the ordinary-subnet rule.

The /27 block size is 32. The subnet containing 192.168.10.142 runs from .128 through .159, giving:

• Network: 192.168.10.128

• Usable host range: 192.168.10.129–192.168.10.158

• Broadcast: 192.168.10.159

In general, read the CIDR prefix to determine the mask and number of host bits. Use the host-bit count to find the subnet size, then identify the network address with host bits all zero, the broadcast address with host bits all one, and the usable range between them. Remember the /31 point-to-point and /32 exceptions.