Network Address Classes | Class A, B, C, D, E

The Internet community originally defined five address classes to accommodate networks of varying sizes. Microsoft TCP/IP supports class A, B, and C addresses assigned to hosts. The class of address defines which bits are used for the network ID and which bits are used for the host ID. It also defines the possible number of networks and the number of hosts per network.

Class A

Class A addresses are assigned to networks with a very large number of hosts. The high-order bit in a class A address is always set to zero. The next seven bits (completing the first octet) complete the network ID. The remaining 24 bits (the last three octets) represent the host ID. This allows for 126 networks and 16,777,214 hosts per network. Figure 1.4 illustrates the structure of class A addresses.
Figure 1.4 Class A IP Addresses

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Class B

Class B addresses are assigned to medium-sized to large-sized networks. The two high-order bits in a class B address are always set to binary 1 0. The next 14 bits (completing the first two octets) complete the network ID. The remaining 16 bits (last two octets) represent the host ID. This allows for 16,384 networks and 65,534 hosts per network. Figure 1.5 illustrates the structure of class B addresses.
Figure 1.5 Class B IP Addresses

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Class C

Class C addresses are used for small networks. The three high-order bits in a class C address are always set to binary 1 1 0. The next 21 bits (completing the first three octets) complete the network ID. The remaining 8 bits (last octet) represent the host ID. This allows for 2,097,152 networks and 254 hosts per network. Figure 1.6 illustrates the structure of class C addresses.
Figure 1.6 Class C IP Addresses

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Class D

Class D addresses are reserved for IP multicast addresses. The four high-order bits in a class D address are always set to binary 1 1 1 0. The remaining bits are for the address that interested hosts recognize. Microsoft supports class D addresses for applications to multicast data to multicast-capable hosts on an internetwork.

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Class E

Class E is an experimental address that is reserved for future use. The high-order bits in a class E address are set to 1111.
Table 1.11 is a summary of address classes A, B, and C that can be used for host IP addresses.
Table 1.11 IP Address Class Summary

Class	Value for w 1	Network ID Portion	Host ID Portion	Available Networks	Hosts per Network
A	1–126	w	x.y.z	126	16,777,214
B	128–191	w.x	y.z	16,384	65,534
C	192–223	w.x.y	z	2,097,152	254
1 The class A address 127 .x.y.z is reserved for loopback testing and interprocess communication on the local computer.

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Network ID Guidelines

The network ID identifies the TCP/IP hosts that are located on the same physical network. All hosts on the same physical network must be assigned the same network ID to communicate with each other.
Follow these guidelines when assigning a network ID:

• The network ID must be unique to the IP internetwork. If you plan on having a direct routed connection to the public Internet, the network ID must be unique to the Internet. If you do not plan on connecting to the public Internet, the local network ID must be unique to your private internetwork.
  
• The network ID cannot begin with the number 127. The number 127 in a class A address is reserved for internal loopback functions.
  
• All bits within the network ID cannot be set to 1. All 1's in the network ID are reserved for use as an IP broadcast address.
  
• All bits within the network ID cannot be set to 0. All 0's in the network ID are used to denote a specific host on the local network and are not routed.
  

Table 1.12 lists the valid ranges of network IDs based on the IP address classes. To denote IP network IDs, the host bits are all set to 0. Note that even though expressed in dotted decimal notation, the network ID is not an IP address.
Table 1.12 Class Ranges of Network IDs

Address Class	First Network ID	Last Network ID
Class A	1.0.0.0	126.0.0.0
Class B	128.0.0.0	191.255.0.0
Class C	192.0.0.0	223.255.255.0

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Host ID Guidelines

The host ID identifies a TCP/IP host within a network. The combination of IP network ID and IP host ID is an IP address.
Follow these guidelines when assigning a host ID:

• The host ID must be unique to the network ID.
  
• All bits within the host ID cannot be set to 1 because this host ID is reserved as a broadcast address to send a packet to all hosts on a network.
  
• All bits in the host ID cannot be set to 0 because this host ID is reserved to denote the IP network ID.
  

Table 1.13 lists the valid ranges of host IDs based on the IP address classes.
Table 1.13 Class Ranges of Host IDs

Address Class	First Host ID	Last Host ID
Class A	w .0.0.1	w .255.255.254
Class B	w.x .0.1	w.x .255.254
Class C	w.x.y. 1	w.x.y .254

IP Addresses Concepts

Introduction

Every machine on the the Internet has a unique number assigned to it, called an IP address. Without a unique IP address on your machine, you will not be able to communicate with other devices, users, and computers on the Internet. You can look at your IP address as if it were a telephone number, each one being unique and used to identify a way to reach you and only you.

IPv4 and IPv6 Addresses

There are two flavors of IP Addresses that can be used on a network. The first, and the version that the Internet and most routers are currently configured for, is IPv4 or Internet Protocol version 4. This version uses 32-bit addresses, which limits the amount of addresses to 4,294,967,296 possible unique addresses. Some of these addresses, about 290 million, are also reserved for special purposes.

Due to the popular growth of the Internet there has been concern that the pool of possible addresses would be exhausted in the near future. With this in mind, a new version of IP addresses was developed called IPv6, or Internet Protocol version 6, that would change the address size from 32-bit address to 128-bit addresses.

This change would allow for generous IP address allocations to networks without any foreseeable problem with the amount of addresses available. In order to use IPv6 addresses, though, existing routers and hardware would need to be upgraded or configured to use this new version of IP addresses.

As IPv4 is still the most commonly used, this tutorial will be focusing on that version of IP addresses.

The Address Itself

An IP address always consists of 4 numbers separated by periods, with the numbers having a possible range of 0 through 255. An example of how an IP address appears is: 192.168.1.10
This representation of an IP address is called decimal notation and is what is generally used by humans to refer to an IP address for readability purposes.

With the ranges for each number being between 0 and 255 there are a total 4,294,967,296 possible IP addresses.Out of these addresses there are 3 special ranged that are reserved for special purposes. The first is the 0.0.0.0 address and refers to the default network and the 255.255.255.255 address which is called the broadcast address.

These addresses are used for routing, which will not be covered in this tutorial. The third address, 127.0.0.1, is the loopback address, and refers to your machine. Whenever you see, 127.0.0.1, you are actually referring to your own machine.

That means if you clicked on this link, http://127.0.0.1, you are actually trying to connect to your own computer, and unless you have a web server running, you will get a connection error.

There are some guidelines to to how IP address can appear, though. The four numbers must be between 0 and 255, and the IP address of 0.0.0.0 and 255.255.255.255 are reserved, and are not considered usable IP addresses.

IP addresses must be unique for each computer connected to a network. That means that if you have two computers on your network, each must have a different IP address to be able to communicate with each other. If by accident the same IP address is assigned to two computers, then those computers would have what is called an "IP Conflict" and not be able to communicate with each other.

IP address classes

These IP addresses can further be broken down into classes. These classes are A, B, C, D, E and their possible ranges can be seen in Figure 2 below.

Class	Start address	Finish address
A	0.0.0.0	126.255.255.255
B	128.0.0.0	191.255.255.255
C	192.0.0.0	223.255.255.255
D	224.0.0.0	239.255.255.255
E	240.0.0.0	255.255.255.255

Figure 2. IP address Classes

If you look at the table you may notice something strange. The range of IP address from Class A to Class B skips the 127.0.0.0-127.255.255.255 range. That is because this range is reserved for the special addresses called Loopback addresses that have already been discussed above.

The rest of classes are allocated to companies and organizations based upon the amount of IP addresses that they may need. Listed below are descriptions of the IP classes and the organizations that will typically receive that type of allocation.

Default Network: The special network 0.0.0.0 is generally used for routing.

Class A: From the table above you see that there are 126 class A networks. These networks consist of 16,777,214 possible IP addresses that can be assigned to devices and computers. This type of allocation is generally given to very large networks such as multi-national companies.

Loopback: This is the special 127.0.0.0 network that is reserved as a loopback to your own computer. These addresses are used for testing and debugging of your programs or hardware.

Class B: This class consists of 16,384 individual networks, each allocation consisting of 65,534 possible IP addresses. These blocks are generally allocated to Internet Service Providers and large networks, like a college or major hospital.

Class C: There is a total of 2,097,152 Class C networks available, with each network consisting of 255 individual IP addresses. This type of class is generally given to small to mid-sized companies.

Class D: The IP addresses in this class are reserved for a service called Multicast.

Class E: The IP addresses in this class are reserved for experimental use.

Broadcast: This is the special network of 255.255.255.255, and is used for broadcasting messages to the entire network that your computer resides on.

Private Addresses

There are also blocks of IP addresses that are set aside for internal private use for computers not directly connected to the Internet. These IP addresses are not supposed to be routed through the Internet, and most service providers will block the attempt to do so.

These IP addresses are used for internal use by company or home networks that need to use TCP/IP but do not want to be directly visible on the Internet. These IP ranges are:

Class	Private Start Address	Private End Address
A	10.0.0.0	10.255.255.255
B	172.16.0.0	172.31.255.255
C	192.168.0.0	192.168.255.255

If you are on a home/office private network and want to use TCP/IP, you should assign your computers/devices IP addresses from one of these three ranges.

That way your router/firewall would be the only device with a true IP address which makes your network more secure.

Common Problems and Resolutions

The most common problem people have is by accident assigning an IP address to a device on your network that is already assigned to another device. When this happens, the other computers will not know which device should get the information, and you can experience erratic behavior.

On most operating systems and devices, if there are two devices on the local network that have the same IP address, it will generally give you a "IP Conflict" warning. If you see this warning, that means that the device giving the warning, detected another device on the network using the same address.

The best solution to avoid a problem like this is to use a service called DHCP that almost all home routers provide. DHCP, or Dynamic Host Configuration Protocol, is a service that assigns addresses to devices and computers.

You tell the DHCP server what range of IP addresses you would like it to assign, and then the DHCP server takes the responsibility of assigning those IP addresses to the various devices and keeping track so those IP addresses are assigned only once.

Conclusion

IP addresses and their function on the Internet is an important concept to understand. Hopefully with this tutorial you will have a firmer grasp on this concept, which should help you troubleshoot problems and bring a better understanding of how the Internet works.