IP Addressing

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An IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network. An IP address is a 32 bit binary number usually represented as 4 decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by decimal points. This is known as "dotted decimal" notation.

Example: 140.179.220.200

It is sometimes useful to view the values in their binary form.

140     .179     .220     .200
10001100.10110011.11011100.11001000

Every IP address consists of two parts, one identifying the network and one identifying the node. The Class of the address and the subnet mask determine which part belongs to the network address and which part belongs to the node address.

Address Classes

There are 5 different address classes. You can determine which class any IP address is in by examining the first 4 bits of the IP address.

• Class A addresses begin with 0xxx, or 1 to 126 decimal.
• Class B addresses begin with 10xx, or 128 to 191 decimal.
• Class C addresses begin with 110x, or 192 to 223 decimal.
• Class D addresses begin with 1110, or 224 to 239 decimal.
• Class E addresses begin with 1111, or 240 to 254 decimal.

Addresses beginning with 0111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine. [You can test this: you should always be able to ping 127.0.0.1, which points to yourself] Class D addresses are reserved for multicasting. Class E addresses are reserved for future use. They should not be used for host addresses.
Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N) and which part belongs to the node (n).

• Class A -- NNNNNNNN.nnnnnnnn.nnnnnnn.nnnnnnn
• Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn
• Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn

In the example, 140.179.220.200 is a Class B address so by default the Network part of the address (also known as the Network Address) is defined by the first two octets (140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200).

In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, 140.179.0.0 specifies the network address for 140.179.220.200. When the node section is set to all "1"s, it specifies a broadcast that is sent to all hosts on the network. 140.179.255.255 specifies the example broadcast address. Note that this is true regardless of the length of the node section.

Subnetting

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Subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used to connect IP networks to minimize the amount of traffic each segment must receive.

Subnet Masking

Applying a subnet mask to an IP address allows you to identify the network and node parts of the address. Performing a bitwise logical AND operation between the IP address and the subnet mask results in the Network Address or Number.
For example, using our test IP address and the default Class B subnet mask, we get:

10001100.10110011.11110000.11001000      140.179.240.200   Class B IP Address
11111111.11111111.00000000.00000000      255.255.000.000   Default Class B Subnet Mask
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10001100.10110011.00000000.00000000      140.179.000.000   Network Address

Default subnet masks:

• Class A - 255.0.0.0 - 11111111.00000000.00000000.00000000
• Class B - 255.255.0.0 - 11111111.11111111.00000000.00000000
• Class C - 255.255.255.0 - 11111111.11111111.11111111.00000000

Allowed Class A Subnet and Host IP addresses

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# bits	Subnet Mask	# Subnets	# Hosts	Nets * Hosts
2	255.192.0.0	2	4194302	8388604
3	255.224.0.0	6	2097150	12582900
4	255.240.0.0	14	1048574	14680036
5	255.248.0.0	30	524286	15728580
6	255.252.0.0	62	262142	16252804
7	255.254.0.0	126	131070	16514820
8	255.255.0.0	254	65534	16645636
9	255.255.128.0	510	32766	16710660
10	255.255.192.0	1022	16382	16742404
11	255.255.224.0	2046	8190	16756740
12	255.255.240.0	4094	4094	16760836
13	255.255.248.0	8190	2046	16756740
14	255.255.252.0	16382	1022	16742404
15	255.255.254.0	32766	510	16710660
16	255.255.255.0	65534	254	16645636
17	255.255.255.128	131070	126	16514820
18	255.255.255.192	262142	62	16252804
19	255.255.255.224	524286	30	15728580
20	255.255.255.240	1048574	14	14680036
21	255.255.255.248	2097150	6	12582900
22	255.255.255.252	4194302	2	8388604

Allowed Class B Subnet and Host IP addresses

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# bits	Subnet Mask	# Subnets	# Hosts	Nets * Hosts
2	255.255.192.0	2	16382	32764
3	255.255.224.0	6	8190	49140
4	255.255.240.0	14	4094	57316
5	255.255.248.0	30	2046	61380
6	255.255.252.0	62	1022	63364
7	255.255.254.0	126	510	64260
8	255.255.255.0	254	254	64516
9	255.255.255.128	510	126	64260
10	255.255.255.192	1022	62	63364
11	255.255.255.224	2046	30	61380
12	255.255.255.240	4094	14	57316
13	255.255.255.248	8190	6	49140
14	255.255.255.252	16382	2	32764

Allowed Class C Subnet and Host IP addresses

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# bits	Subnet Mask	# Subnets	# Hosts	Nets * Hosts
2	255.255.255.192	2	62	124
3	255.255.255.224	6	30	180
4	255.255.255.240	14	14	196
5	255.255.255.248	30	6	180
6	255.255.255.252	62	2	124

Here is another, more detailed, example. Say you are assigned a Class C network number of 200.133.175.0 .
You want to utilize this network across multiple small groups within an organization. You can do this by subnetting that network with a subnet address.

We will break this network into 14 subnets of 14 nodes each. This will limit us to 196 nodes on the network instead of the 254 we would have without subnetting, but gives us the advantages of traffic isolation and security. To accomplish this, we need to use a subnet mask 4 bits long.
Recall that the default Class C subnet mask is

255.255.255.0 (11111111.11111111.11111111.00000000 binary)

Extending this by 4 bits yeilds a mask of

255.255.255.240 (11111111.11111111.11111111.11110000 binary)

This gives us 16 possible network numbers, 2 of which cannot be used: