What is an IP Address?

An Internet Protocol (IP) address is a numeric label consisting of a 32 bit number assigned to a network capable device that uses IP for communication. The address fundamentally serves two purposes: location addressing and computer host or network interface identification. The address indicates where the connected device resides with the majority of hosts/devices still using the IPv4 (Internet Protocol Version 4) form of addressing. A significant limitation of the legacy IPv4 addressing is that it supports less than 4.3 billion total addresses. Based on the rapid growth of the Internet and related technologies, the use of IPv4 is not sustainable for the long term. In the mid-1990’s, the new IPv6 technique was developed which makes use of 128 bits for the IP address. IPv6 technology continues to be deployed, albeit slowly. The Internet Assigned Numbers Authority (IANA) is responsible under the IETF for management of the IP address space allocation globally. Beneath the IANA, there are five regional Internet registries (RIRs) that are responsible for allocating IP address blocks to Internet service providers (ISPs) and other trusted organizations.

IP address classes

There were five IP address classes in use before the majority of industry switched to classless routing. There were A, B, C, D, and E. Class A addresses were used for networks with a very large number of total hosts. Class B was designed for use on medium to large networks, and C for small local area networks (LANs). Class D and E were set aside for multicast and experimental purposes. In the following table, the four octets that make up an IP address (a, b, c, and d respectfully) are displayed in how they were distributed in classes A, B, and C. classes A, B, and C.

Class	1st Octet Decimal Range	1st Octet High Order Bits	Network/Host ID (N=Network, H=Host)	Default Subnet Mask	Number of Networks	Hosts per Network (Usable Addresses)
A	1 – 126*	0	N.H.H.H	255.0.0.0	126 (27 – 2)	16,777,214 (224 – 2)
B	128 – 191	10	N.N.H.H	255.255.0.0	16,382 (214 – 2)	65,534 (216 – 2)
C	192 – 223	110	N.N.N.H	255.255.255.0	2,097,150 (221 – 2)	254 (28 – 2)
D	224 – 239	1110	Reserved for Multicasting
E	240 – 254	1111	Experimental; used for research