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
- How many subnets?
2(masked bits) - 2 = Subnets
- How many valid hosts per subnet?
2(unmasked bits) - 2 = Hosts
- What are the valid subnets?
256-(subnet base)=Base number
- What are the valid hosts in the subnets?
All numbers between subnets minus the all 1s (.255) and
all 0s (.0) host addresses.
- 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)
- (22)-2 = 2 Subnets
- (26)-2 = 62 Hosts per subnet
- 256 - 192 = 64 (.01000000) {For the first
subnet}
- 65 to 126 (.01000001 to .01111110)
Valid hosts in the subnets
- 127 (.01111111) Broadcast
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