ZyXEL G-560 User’s Guide
F-2 IP Subnetting
A class “A” address (24 host bits) can have 2
24
–2 hosts (approximately 16 million hosts).
Since the first octet of a class “A” IP address must contain a “0”, the first octet of a class “A” address can
have a value of 0 to 127.
Similarly the first octet of a class “B” must begin with “10”, therefore the first octet of a class “B” address
has a valid range of 128 to 191. The first octet of a class “C” address begins with “110”, and therefore has a
range of 192 to 223.
Chart F-2 Allowed IP Address Range By Class
CLASS
ALLOWED RANGE OF FIRST OCTET
(BINARY)
ALLOWED RANGE OF FIRST OCTET
(DECIMAL)
Class A
0
0000000 to
0
1111111
0 to 127
Class B
10
000000 to
10
111111
128 to 191
Class C
110
00000 to
110
11111
192 to 223
Class D
1110
0000 to
1110
1111
224 to 239
Subnet Masks
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). A subnet mask has 32 bits; each bit of the mask corresponds
to a bit of the IP address. 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.
Subnet masks are expressed in dotted decimal notation just as IP addresses are. The “natural” masks for
class A, B and C IP addresses are as follows.
Chart F-3 “Natural” Masks
CLASS NATURAL
MASK
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0
Subnetting
With subnetting, the class arrangement of an IP address is ignored. For example, a class C address no
longer has to have 24 bits of network number and 8 bits of host ID. With subnetting, some of the host ID
bits are converted into network number bits. By convention, subnet masks always consist of a continuous