1

IP Addressing: Classful IP Addressing: Classful

0network host

10 network host

110 network host

1110 multicast address

A

B

C

D

class1.0.0.0 to127.255.255.255

128.0.0.0 to191.255.255.255

192.0.0.0 to223.255.255.255

224.0.0.0 to239.255.255.255

32 bits

0 8 16 24 31

Dotted-decimal notation 158.108.21.9

Binary form 10011110 01101100 00010101 00001001

2

IP Addresses: IP Addresses: ExamplesExamples

Class “A” address: www.mit.edu18.181.0.31

(18<128 => Class A)

Class “B” address: cs.sci.ku.ac.th158.108.21.9

(128<158<128+64 => Class B)

3

SubnettingSubnetting

CLASS “B”e.g.

Company

10 Net ID Host-ID

2 14 16

10 Net ID Host-ID

2 14 16

0000

Subnet ID (20) SubnetHost ID (12)

10 Net ID Host-ID

2 14 16

1111

Subnet ID (20) SubnetHost ID (12)

10 Net ID Host-ID

2 14 16

000000

Subnet ID (22) SubnetHost ID (10)

10 Net ID Host-ID

2 14 16

1111011011

Subnet ID (26) SubnetHost ID (6)

e.g. Site

e.g. Dept

4

Subnet MaskSubnet Mask Subnet mask: All 1’s for network suffix and All

0’s for host suffix. (เช่�น 255255255. . .192)

Used for routing table lookup: IF ((Mask[i] & Destination Addr) = = Destination[i])

Forward to NextHop[i] Mask must have contiguous 1s followed by

contiguous zeros. Subnet mask can end on any bit.

Internal routers & hosts use subnet mask to identify “subnet ID” and route packets between “subnets” within the “network”.

Routers do not support other types of masks.