The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP "Classful" (Conventional) Addressing

IP Addresses With Special Meanings 1 2 3 IP Multicast Addressing

In the preceding topic I showed how certain IP addresses cannot be used for regular network devices because they are addresses with “special meanings”. These special addresses reduce the total number of networks that are possible in the “classful” addressing scheme, and also the total number of hosts available in each network.

In addition to these unusable numbers, there are several other sets of IP addresses set aside for various special uses, which are not available for normal address assignment. These ranges of IP addresses generally fall into the following three categories: reserved, loopback and private addresses.

Several blocks of addresses were designated just as “reserved” with no specific indication given of what they were reserved for. They may have been set aside for future experimentation, or for internal use in managing the Internet, or for other purposes. (In general, it’s a good idea to set aside some portion of any limited resource for unanticipated needs!) There are a couple of these blocks in each of the three main classes (A, B, and C), appearing right at the beginning and end of each class. (In a manner of speaking, all of classes D and E are also “reserved”, since they aren't used for regular addressing, though the term “reserved” is usually used to refer to unusable parts of classes A, B and C.

Normally, when a TCP/IP application wants to send information, that information travels down the protocol layers to IP where it is encapsulated in an IP datagram. That datagram then passes down to the data link layer of the device's physical network for transmission to the next hop, on the way to the IP destination.

However, one special range of addresses is set aside for loopback functionality. This is the range 127.0.0.0 to 127.255.255.255. IP datagrams sent by a host to a 127.x.x.x loopback address are not passed down to the data link layer for transmission. Instead, they “loop back” to the source device at the IP level. In essence, this represents a “short-circuiting” of the normal protocol stack; data is sent by a device's layer three IP implementation and then immediately received by it.

The purpose of the loopback range is testing of the TCP/IP protocol implementation on a host. Since the lower layers are short-circuited, sending to a loopback address allows the higher layers (IP and above) to be effectively tested without the chance of problems at the lower layers manifesting themselves. 127.0.0.1 is the address most commonly used for testing purposes.

IP Addresses With Special Meanings 1 2 3 IP Multicast Addressing