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Appendix B IP Addresses and Subnetting

By convention, subnet masks always consist of a continuous sequence of ones beginning from the leftmost bit of the mask, followed by a continuous sequence of zeros, for a total number of 32 bits.

Subnet masks can be referred to by the size of the network number part (the bits with a “1” value). For example, an “8-bit mask” means that the first 8 bits of the mask are ones and the remaining 24 bits are zeroes.

Subnet masks are expressed in dotted decimal notation just like IP addresses. The following examples show the binary and decimal notation for 8-bit, 16-bit, 24-bit and 29-bit subnet masks.

Table 51 Subnet Masks

	BINARY
	1ST	2ND	3RD	4TH	DECIMAL
	OCTET	OCTET	OCTET	OCTET
8-bit mask	11111111	00000000	00000000	00000000	255.0.0.0

16-bit mask	11111111	11111111	00000000	00000000	255.255.0.0

24-bit mask	11111111	11111111	11111111	00000000	255.255.255.0

29-bit mask	11111111	11111111	11111111	11111000	255.255.255.248

Network Size

The size of the network number determines the maximum number of possible hosts you can have on your network. The larger the number of network number bits, the smaller the number of remaining host ID bits.

An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 with a 24-bit subnet mask, for example). An IP address with host IDs of all ones is the broadcast address for that network (192.168.1.255 with a 24-bit subnet mask, for example).

As these two IP addresses cannot be used for individual hosts, calculate the maximum number of possible hosts in a network as follows:

Table 52 Maximum Host Numbers

SUBNET MASK		HOST ID SIZE		MAXIMUM NUMBER OF
SUBNET MASK		HOST ID SIZE		HOSTS
				HOSTS
8 bits	255.0.0.0	24 bits	224 – 2	16777214
16 bits	255.255.0.0	16 bits	216 – 2	65534
24 bits	255.255.255.0	8 bits	28 – 2	254
29 bits	255.255.255.248	3 bits	23 – 2	6

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