ZyXEL Communications P-792H User Manual page 447

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 147 Subnet Masks
8-bit mask
16-bit
mask
24-bit
mask
29-bit
mask
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 148 Maximum Host Numbers
SUBNET MASK
8 bits 255.0.0.0
16 bits 255.255.0.0
24 bits 255.255.255.0
29 bits 255.255.255.2
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P-792H v2 User's Guide
BINARY
1ST 2ND
OCTET OCTET
11111111 00000000
11111111 11111111
11111111 11111111
11111111 11111111
HOST ID SIZE
24 bits
16 bits
8 bits
3 bits
Appendix E IP Addresses and Subnetting
3RD 4TH
OCTET OCTET
00000000 00000000
00000000 00000000
11111111 00000000
11111111 11111000
MAXIMUM NUMBER OF
HOSTS
24
2 – 2 16777214
16
2 – 2 65534
8
2 – 2 254
3
2 – 2 6
DECIMAL
255.0.0.0
255.255.0.0
255.255.255.0
255.255.255.24
8
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