Different Types Of IP Version 4 Addresses
IPv4 Address Classes and Default Subnet Masks
In my previous article, I have talked about some of the facts that you need to know about IP Version four addresses. In this article, I want to look at some of the different types of IP Version 4 addresses. There are various types and classes of IPv4 addresses. While address classes are becoming less important in networking, they are still used and referred to commonly in network documentation.
In 1981, IPv4 addresses were assigned using classful addressing as defined in RFC 790. Customers were allocated a network address based on one of three classes, A, B, or C. The RFC divided the unicast ranges into specific classes:
- Class A (0.0.0.0/8 to 127.0.0.0/8) – Designed to support extremely large networks with more than 16 million host addresses. It used a fixed /8 prefix with the first octet to indicate the network address and the remaining three octets for host addresses.
- Class B (18.104.22.168 /16 – 22.214.171.124 /16) – Designed to support the needs of moderate to large size networks with up to approximately 65,000 host addresses. It used a fixed /16 prefix with the two high-order octets to indicate the network address and the remaining two octets for host addresses.
- Class C (192.0.0.0 /24 – 126.96.36.199 /24) – Designed to support small networks with a maximum of 254 hosts. It used a fixed /24 prefix with the first three octets to indicate the network and the remaining octet for the host addresses.
Note: There is also a Class D multicast block consisting of 188.8.131.52 to 184.108.40.206 and a Class E experimental address block consisting of 240.0.0.0 – 255.0.0.0.
As shown in the figure, the classful system allocated 50% of the available IPv4 addresses to 128 Class A networks, 25% of the addresses to Class B and then Class C shared the remaining 25% with Class D and E. Although appropriate at the time, as the internet grew it was obvious that this method was wasting addresses and depleting the number of available IPv4 network addresses.
The diagram is a pie chart showing the percentage of Class A, B, C, D, & E IPv4 addressing with the total number of networks and hosts per class A, B, and C networks. Percentages are: class A = 50%, class B = 25%, class C = 12.5%, and class D and E = 12.5%. For the total number of networks and total number of hosts per network: class A = 128 networks with 16,777,214 total hosts per network; class B = 16,384 networks with 65,534 total hosts per network; and class C = 2,097,152 networks with 254 total hosts per network.
Classful addressing was abandoned in the late 1990s for the newer and current classless addressing system. However, as we will see later, classless addressing was only a temporary solution to the depletion of IPv4 addresses.
Reserved Private Addresses
Public IPv4 addresses are addresses that are globally routed between ISP routers. However, not all available IPv4 addresses can be used on the internet. There are blocks of addresses called private addresses that are used by most organizations to assign IPv4 addresses to internal hosts.
In the mid-1990s, private IPv4 addresses were introduced because of the depletion of IPv4 address space. Private IPv4 addresses are not unique and can be used by any internal network.
These are the private address blocks:
- 10.0.0.0 /8 or 10.0.0.0 to 10.255.255.255
- 172.16.0.0 /12 or 172.16.0.0 to 172.31.255.255
- 192.168.0.0 /16 or 192.168.0.0 to 192.168.255.255
It is important to know that addresses within these address blocks are not allowed on the internet and must be filtered (discarded) by internet routers. For example, as shown in the figure, users in networks 1, 2, or 3 are sending packets to remote destinations. The ISP routers would see that the source IPv4 addresses in the packets are from private addresses and would, therefore, discard the packets.
The figure shows three separate networks, network 1 at 10 dot 0 dot 0 dot 0 / 8, network 2 at 172 dot 16 dot 0 dot 0 / 16, and network 3 at 192 dot 168 dot 0 dot 0. Each network router connects to its own i s p router. The i s p routers are connected to the internet. The networks are privately addressed. In the figure, each router is sending a privately addressed packet to its i s p router. Each i s p router has a red X over the link signifying that the router will not accept the packet.
Private Addresses Cannot be Routed over the Internet
Most organizations use private IPv4 addresses for their internal hosts. However, these RFC 1918 addresses are not routable on the internet and must be translated to a public IPv4 address. Network Address Translation (NAT) is used to translate between private IPv4 and public IPv4 addresses. This is usually done on the router that connects the internal network to the ISP’s network.
Home routers provide the same capability. For instance, most home routers assign IPv4 addresses to their wired and wireless hosts from the private address of 192.168.1.0 /24. The home router interface that connects to the Internet service provider (ISP) network is often assigned a public IPv4 address to use on the internet.
I know you might agree with some of the points that I have raised in this article. You might not agree with some of the issues raised. Let me know your views about the topic discussed. We will appreciate it if you can drop your comment. Thanks in anticipation.
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