ZyXEL Communications WAP3205 User Manual page 89

How much of the IP address is the network number and how much is the host ID varies according to the
subnet mask.
Sub ne t Ma sks
A subnet mask is used to determine which bits are part of the network number, and which bits are part
of the host ID (using a logical AND operation). The term "subnet" is short for "sub-network".
A subnet mask has 32 bits. If a bit in the subnet mask is a "1" then the corresponding bit in the IP address
is part of the network number. If a bit in the subnet mask is "0" then the corresponding bit in the IP
address is part of the host ID.
The following example shows a subnet mask identifying the network number (in bold text) and host ID of
an IP address (192.168.1.2 in decimal).
Table 38 IP Address Network Number and Host ID Example
IP Address (Binary)
Subnet Mask (Binary)
Network Number
Host ID
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 39 Subnet Masks
8-bit mask
16-bit mask
24-bit mask
29-bit mask
Ne two rk 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).
Appendix A IP Addresses and Subnetting
1ST O C T ET :
(192)
11000000
11111111
11000000
BINARY
1ST O C T ET 2ND O C T ET 3RD O C T ET
11111111 00000000
11111111 11111111
11111111 11111111
11111111 11111111
WAP3205 v3 User's Guide
2ND O C T ET : 3RD O C T ET :
(168) (1)
10101000 00000001
11111111 11111111
10101000 00000001
4T H O C T ET
00000000 00000000
00000000 00000000
11111111 00000000
11111111 11111000
89
4T H O C T ET
(2)
00000010
00000000
00000010
DEC IMAL
255.0.0.0
255.255.0.0
255.255.255.0
255.255.255.248