ZyXEL Communications STB-1001S2 Notation

Appendix B IP Addresses and Subnetting

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 15 Subnet Masks

	BINARY
	1ST	2ND	3RD	4TH OCTET	DECIMAL
	OCTET	OCTET	OCTET	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

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 16 Maximum Host Numbers

SUBNET MASK		HOST ID SIZE		MAXIMUM NUMBER OF 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

Notation

Since the mask is always a continuous number of ones beginning from the left, followed by a continuous number of zeros for the remainder of the 32 bit mask, you can simply specify the number of ones instead of writing the value of each octet. This is usually specified by writing a “/” followed by the number of bits in the mask after the address.

For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with subnet mask 255.255.255.128.

The following table shows some possible subnet masks using both notations.

Table 17 Alternative Subnet Mask Notation

SUBNET MASK	ALTERNATIVE	LAST OCTET	LAST OCTET
SUBNET MASK	NOTATION	(BINARY)	(DECIMAL)
	NOTATION	(BINARY)	(DECIMAL)
255.255.255.0	/24	0000 0000	0

255.255.255.128	/25	1000 0000	128

	65
STB-1001S2 User’s Guide	65