ZyXEL Communications MWR102 manual Subnet Mask Identifying Network Number, Subnet Masks

Table 26 Subnet Mask - Identifying Network Number

	1ST	2ND	3RD	4TH
	OCTET:	OCTET:	OCTET:	OCTET
	(192)	(168)	(1)	(2)
IP Address (Binary)	11000000	10101000	00000001	00000010
Subnet Mask (Binary)	11111111	11111111	11111111	00000000
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 27 Subnet Masks

	BINARY
					DECIMAL
	1ST	2ND	3RD	4TH
	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

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