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IP Addressing

Related Articles

Subnetting IPv4
IPv4 Notes
IPv6 Notes


The network layer, (Layer 3) of the OSI Model defines each device on the network. In IPv4, each packet uses a 32-bit source address and a 32-bit destination address in the Layer 3 header. These addresses are represented in a dotted decimal format.

IPv4 addresses are easier to remember while in the dotted decimal format rather than in their natural binary state. For example, take the address This is a lot easier to remember than: 10101100000100000000010000010100 is it not?

Network and Hosts

There are two parts to IPv4 address, the Network and the Host portions. The network portion of an IPv4 address is known as the high-order bits because they are the most significant. A group of hosts that have identical bit patterns is known as the network. For example, take these two addresses: and The portion of the IP address in bold represents the network portion of the address, the portion in black is the host part of the address.

IPv4 Network Classes

There are five IPv4 Network classes, (A, B, C, D & E,) but only the three most used, (A, B & C) will be focused on. Class D is a Multicast address while Class E is used for experimental and future use. Here are Classes A, B & C's basic information. A Class A, B or C address space is known as Classful Addressing.

Classful & Classless Addressing

Classful Addressing

This addressing method divides the address space for Internet Protocol Version 4 (IPv4) into five address classes. Each class, coded in the first four bits of the address, defines either a different network size. In other words, each size of the network is defined and has a "Class" associated with it.

Classless Addressing

Classless addressing uses a variable number of bits for the network and host portions of the address. They are not a fixed size and do not have a particular "Class" associated with it.

Class 1st Octet Range Prefix & Mask # of Networks # of Hosts
A 1 to 127 /8 126 (27) 16,777,214 (224 -2)
B 128 to 191 /16 16,384 (214) 65,534 (216 -2)
C 192 to 223 /24 2,097,159(221) 254 (28 -2)

IPv4 Class Address Blocks

Class A Blocks

The Class A Block was designed to support large networks with more than 16 million host addresses.

Class B Blocks

The Class B Block was designed to support moderate to large networks with more than 65,000 host addresses.

Class C Blocks

The Class C Block was designed to support small networks with a maximum of 254 host addresses.


Like IPv4, IPv6 is used for carrying data in packets from a source to a destination over various networks. It can support very large numbers of nodes allowing for 2128 possible nodes or address combinations.

With IPv6, each device can have its own separate IP address and with this possibility provides more security than IPv4 can.

There are other benefits to using IPv6 over IPv4. These benefits include:
(For more about IPv6, read the IPv6 Notes under "Related Articles".)

  • Extended Address Space: IPv6 provides 128-bit source and destination addresses compared to 32-bit addresses with IPv4. This represents an extremely large, to put it lightly, number of addresses, 2128 or roughly 340 trillion, trillion, trillion addresses.
  • Stateless Autoconfiguration: IPv6 provides a configuration mechanism where hosts can self-generate a routable table. IPv4 autoconfigured addresses are useable only on the local subnet and are never forwarded by a router. Stateful autoconfiguration using DHCPv6 is an option in IPv6.
  • No need for NAT or PAT: Because of the large number of public addresses with IPv6, there is no longer a need for Network Address Translation, (NAT) or Port Address Translation, (PAT). Large Enterprise ISPs can offer each customer a unique IP address for each device the customer has.
  • Broadcasts are no longer needed: IPv6 does not use Layer 3 broadcast addresses, but it does employ solicited node multicast addresses. This is a more efficient and selective technique for processes such as address resolution. IPv4 uses Address Resolution Protocol, (ARP) broadcasts, IPv6 uses solicited node multicasts to accomplish the same purpose.
  • Transistion Tools: A variety of tools accompany IPv6 to help with the transition from IPv4 to IPv6. This includes tunneling and NAT. Tunneling is the process of encapsulating the IPv6 packets into an IPv4 packet so that it can be delivered over IPv4 networks. NAT is the process providing a mechanism to translate an IPv4 address to an IPv6 address or vice versa.