Subnet Masks - ZyXEL Communications ZyWall 10 Reference Manual

ZyWALL 10~100 Series Internet Security Gateway
A class "B" address (16 host bits) can have 2
A class "A" address (24 host bits) can have 2
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.
CLASS ALLOWED RANGE OF FIRST OCTET
Class A
Class B
Class C
Class D

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.
Subnetting
8-2
24
Chart 8-2 Allowed IP Address Range By Class
(BINARY)
00000000 to 01111111
10000000 to 10111111
11000000 to 11011111
11100000 to 11101111
Chart 8-3 "Natural" Masks
CLASS
A
B
C
16
–2 or 65534 hosts.
–2 hosts (approximately 16 million hosts).
ALLOWED RANGE OF FIRST OCTET
NATURAL MASK
255.0.0.0
255.255.0.0
255.255.255.0
(DECIMAL)
0 to 127
128 to 191
192 to 223
224 to 239
IP Subnetting