Инструкция для ZYXEL ZyWALL 2 EE

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ZyWALL 2 Series User’s Guide

I-2

IP Subnetting

A class “A” address (24 host bits) can have 2

–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 I-2 Allowed IP Address Range By Class

CLASS

ALLOWED RANGE OF FIRST OCTET

(BINARY)

ALLOWED RANGE OF FIRST OCTET

(DECIMAL)

Class A

0000000 to

1111111

0 to 127

Class B

000000 to

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 I-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 sequence