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IP Addressing and Subnet Masking

IP addresses are 32 bits long and are represented as a quad octet set.  Each octet represents 1 byte (8 bits) and has a range of 0-255 for 256 values ( 2 8 = 256 ).  IP networks are divided into classes, with each class having a network ID and host range.  Each class uses bits from the first octet as part of the network ID and depending on the network class can use up to two other octets for the network ID with the remaining octets reserved for hosts on the network.  For example, class A networks use the first octet as the network range and the other three octets are for hosts on the networks and class C networks use the first three octets from the left as the network ID, leaving only one octet available for hosts.  The Network classes are divided into classes as follows.

IP Address Classes
Class First Octet's Range Decimal First Octet's Binary Range Network   Host Default Subnet Mask
A
0-126
00000000 - 01111110 Network.Host.Host.Host
Example
10.128.22.100
255.0.0.0
B
128-191
10000000 - 10111111 Network.Network.Host.Host
Example
172.12.15.65
255.255.0.0
C
192-223
11000000 - 11011111 Network.Network.Network.Host
Example
209.38.1.2
255.255.255.0
D
E
224-239
240-255
11100000 - 11111111 D is for multicast
E is experimental
N/A


Some IP addresses are reserved for special use and are not to be used as networks.

Reserved IP Addresses
Address Use
127.0.0.1 Used for loopback
0.0.0.0 Used as default route on Cisco Equipment
255.255.255.255 Broadcast to all nodes on network

IP Network addressing

  • A network address uniquely identifies each network.
  • Every machine on the network shares the same network portion of it's IP address.
  • The node portion of the IP address uniquely identifies the node on its network, can also be called host address.

Identifying the Parts of IP Network Addresses

This table has one example with the other fields left blank for your practice.

Address
Class
Network Portion
Host Portion
Default Subnet
63.125.22.14
A
63.
125.22.14
255.0.0.0
199.2.2.10        
189.88.25.1        
16.12.1.84        
145.1.40.2        
221.220.21.20        


Process For Subnet masking

  1. How many subnets?
    2(masked bits) - 2 = Subnets

  2. How many valid hosts per subnet?
    2(unmasked bits) - 2 = Hosts

  3. What are the valid subnets?
    256-(subnet base)=Base number

  4. What are the valid hosts in the subnets?
    All numbers between subnets minus the all 1s (.255) and all 0s (.0) host addresses.

  5. What is broadcast address of the subnet?
    All the host bits turned on.

  Example of subnet masking

     (255.255.255.192) = (11111111.1111111.1111111.11000000)
  1. (22)-2 = 2 Subnets

  2. (26)-2 = 62 Hosts per subnet

  3. 256 - 192 = 64 (.01000000) {For the first subnet}

  4. 65 to 126 (.01000001 to .01111110) Valid hosts in the subnets

  5. 127 (.01111111) Broadcast


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