IP Addresses

Networks on the Internet

See NFSNet Stats.

IP Address Structure

 The IP Address consists of 32 bits (4 8-bit bytes) They are shown with each byte represented by it's decimal value (0 - 255 (28) separated by decimal points  =(e.g.,
An IP address consists of two parts, one identifying the network and one identifying the node, or host. The Class of the address determines which part belongs to the network address and which part belongs to the node/host address.
e.g. Class A networks assign the first octet to the network and the rest to the hosts. IBM has network number, MIT has network Address Range Num. of Default Type Lowest Highest Networks Mask (1) ________________________________________________________________________ Class A 126 Class B 16 K Class C 2 M Class D Used to support multicasting. Class E Used for experimentation. Reserved 255 Private non-routable IP address: The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets and are non-routable IP address. - (10/8 prefix) - (172.16/12 prefix) - (192.168/16 prefix) Used for home routers 169.254.x.x or 169.254/16 or 144.3.x.x- 'Auto-configuration' (e.g. when DHCP server is not found) 10.x.x.x or 10/8 (Class A) 172.16.x.x - 172.31.x.x or 172.16/12 (Class B) 192.168.x.x or 192.168/16 (Class C) Some standard router addresses and login;password Actiontec (admin) (admin;airlive) 3Com (Actiontec, admin, password) Linksys 192.168 1.1 D-Link (admin;admin) ( Cisco (D-Link) Belkin, root;default Arris (, admin;Atlantis) TP-Link (Comcast) Netgear (Senao admin;admin) Netgear (Samsung) ubee (Comcast) user;user See Router-FAQ.de: Router Adressen & Passworter for more RFC 1918 - Address Allocation for Private Internets RFC 1918 - Address Allocation for Private Internets1. Reserved Addresses: 0 (default route), 127 (loopback address) and 224-255 are reserved. Networks,,, and also are reserved. Broadcast (2) The standard structure can be modified by using a Subnet Mask to use Host Address bits as network addresses locally. Bellcore uses a Subnet Mask of which allocates the 3rd byte to a subnet number and leaves the 4th byte for the host name. This provides more efficient routing of network traffic.
_________________________________________________ RFC 3330 - special use IPv4 addresses.T Also, contrary to popular belief, Microsoft did not invent the use of the 169.254/16 block for auto-config if there is no DHCP server. Apple and other vendors use this block as well. It is defined in this RFC as the Link Local block: - This is the "link local" block. It is allocated for communication between hosts on a single link. Hosts obtain these addresses by auto-configuration, such as when a DHCP server may not be found. >From this RFC, here is the current list of special addresses: Address Block Present Use Reference --------------------------------------------------------------------- "This" Network [RFC1700, page 4] Private-Use Networks [RFC1918] Public-Data Networks [RFC1700, page 181] Cable Television Networks -- Reserved but subject to allocation [RFC1797] Loopback [RFC1700, page 5] Reserved but subject to allocation -- Link Local -- Private-Use Networks [RFC1918] Reserved but subject to allocation -- Reserved but subject to allocation -- Test-Net 6to4 Relay Anycast [RFC3068] Private-Use Networks [RFC1918] Network Interconnect Device Benchmark Testing [RFC2544] Reserved but subject to allocation -- Multicast [RFC3171] Reserved for Future Use [RFC1700, page 4]
Typical Cable Modem Network: PC (IP address assigned by LinkSys) Linksys WRT54GP2 (wireless VoIP router) Linksys BEFT11S4 (wireless 802.11b) Linksys address on Wan side assigned by Optimum Online (cablevision) Optimum Online gateway Cable Modem address (SURFboard SB4100)

As of May of 1995 the Address space was allocated as follows: (2^32 = 4 Billion potential addresses) used as follows: Class A: 38% allocated the remainder is reserved. Class B: 80% in 1993 dropped to 62% in 1995 by reassignment. Class C: 28% allocated.

The weighted total shows that 65.74% (34.74% actually allocated to organizations and 31% reserved for future growth) of the total IPv4 address space has been allocated. It should be noted that careful extrapolations of the current trends suggest that the address space will be exhausted early in the next century. However, the situation is already serious.

Classless Inter-Domain Routing (CIDR) is removing the distinction between classes above. It was introduced in 1993 when the exponential growth if IP address assignment indicated Class B addresses would shortly be depleted. With CIDR, addresses are increasingly thought of as bitwise contiguous blocks of the entire address space.

This scheme allows "supernetting" of addresses together into a blocks which can be advertised as a single routing entry. The practical purpose of this effort is to allow service providers and address registries to delegate realistic address spaces to organizations and be unfettered by the traditional network classes, which were usually inappropriately sized for most organizations. For example the block of 2048 class C network numbers beginning with and ending with can be referenced as 192.24/19, or with a mask of (ie 2^19 in dotted decimal notation). This is referred to as a network with a /19 (19 bit) prefix.

The New Internet Routing and Addressing Architecture (NIMROD) working group of the IETF is working on TCP/IP Version 6 which among other things will make more addresses available.

See IP number assignment statistics, and See appeal to return unused IP numbers.

Example assignments: Internet Assigned Numbers Authority IANA (RESERVED-9) IANA (RESERVED) General Electric Company BBN Planet IANA (RESERVED) Army Information Systems Center IANA (RESERVED-11) Bolt Beranek and Newman Inc. IBM Corporation IANA (RESERVED-6) DoD Intel Information Systems AT&T ITS Xerox Palo Alto Research Center Public Data Network Hewlett-Packard Company Digital Equipment Corporation Apple Computer, Inc. Massachusetts Institute of Technology Ford Motor Company Computer Sciences Corporation Return to Internet History

5/4/95. Send comments and updates to webmaster@cc.bellcore.com