ZyXEL Communications NBG-417N manual 207, Network Size, Subnet Masks

Appendix A IP Addresses and Subnetting

Table 69 Subnet Mask - Identifying Network Number

	1ST	2ND	3RD	4TH
	OCTET:	OCTET:	OCTET:	OCTET
	(192)	(168)	(1)	(2)
Network Number	11000000	10101000	00000001
Host ID				00000010

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 70	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		11111111	11111111	00000000	00000000	255.255.0.0
mask
24-bit		11111111	11111111	11111111	00000000	255.255.255.0
mask
29-bit		11111111	11111111	11111111	11111000	255.255.255.24
mask						8

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).

NBG-417N User’s Guide

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