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These are tables to help in the understanding and generation of VLSM (Variable Length Subnet Masks) networks and CIDR (Classless Inter Domain Routing) which is what the Internet currently uses to route packets on the Internet Backbone.  It makes it possible to represent the millions of networks which form the Internet to be represented by less than 100,000 routes.

Although years ago we couldn't use the "Zero Subnet" now we can, current IP stacks support using this address space.  Same devices may have to be configured Cisco routers for example need the command "ip subnet-zero" put in and while you are at it do "ip classless".


Network Address Ranges by Class
Maximum Number of Hosts by Class
Binary Calculations
Subnet Masks - Binary Representations
CIDR (Classless Inter Domain Routing) Bits to Mask
Classfull Subnetting Tables 


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Network Address Ranges by Class
IP network addresses are issued as follows:

Class Range Default Mask
Class A 1-127 255.0.0.0
Class B 128-191 255.255.0.0
Class C 192-223 255.255.255.0

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Maximum Number of Hosts by Class

Class No. of Addresses
Class A  2^24 16,777,216
Class B 2^16 65,536
Class C 2^8 256

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Binary Calculations

2^0 1 00000001
2^1 2 00000010
2^2 4 00000100
2^3 8 00001000
2^4 16 00010000
2^5 32 00100000
2^6 64 01000000
2^7 128 10000000

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Subnet Masks - Binary Representations

Decimal Binary
.192 11000000 2 bits
.224 11100000 3 bits
.240 11110000 4 bits
.248 11111000 5 bits
.252 11111100 6 bits
.254 11111110 7 bits
.255 11111111 8 bits

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CIDR (Classless Inter Domain Routing) Bits to Mask
This tale helps with notation 192.168.4.4/30

# Bits Subnet Mask
8 255.0.0.0
16 255.255.0.0
17 255.255.128.0
18 255.255.192.0
19 255.255.224.0
20 255.255.240.0
21 255.255.248.0
22 255.255.252.0
23 255.255.254.0
24 255.255.255.0
25 255.255.255.128
26 255.255.255.192
27 255.255.255.224
28 255.255.255.240
29 255.255.255.248
30 255.255.255.252
31 255.255.255.254
32 255.255.255.255

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Classfull Subnetting Tables

Class B      
# of bits Subnet Mask # Subnets # Hosts
2 255.255.192.0 2 16382
3 255.255.224.0 6 8190
4 255.255.240.0 14 4094
5 255.255.248.0 30 2046
6 255.255.252.0 62 1022
7 255.255.254.0 126 510
8 255.255.255.0 254 254
9 255.255.255.128 510 126
10 255.255.255.192 1022 62
11 255.255.255.224 2046 30
12 255.255.255.240 4094 14
13 255.255.255.248 8190 6
14 255.255.255.252 16382 2
Class C      
# Bits Subnet Mask # Subnets # Hosts
24 255.255.255.0 1 254
25 255.255.255.128 2 126
26 255.255.255.192 4 62
27 255.255.255.224 8 30
28 255.255.255.240 16 14
29 255.255.255.248 32 6
30 255.255.255.252 64 2
31 255.255.255.254 UNUSED N/A
32 255.255.255.255 255 1

Last Updated on 8/11/97
By Keith Sinclair
Email: keith@sinclair.org.au
All contents copyright 1997 Keith Sinclair.