What Is a Routing Domain?

A routing domain is also called an autonomous system. It is a small unit that has the right to autonomously decide which routing protocol should be used in this system. A routing domain identifier, sometimes called a domain identifier, refers to an identifier used to identify a routing domain. The routing domain identifier consists of 32 bits. The largest role of the routing domain identifier makes the network divided into independent autonomous systems.

A routing domain identifier is an identifier used to identify a routing domain. The scale of the Internet is very large. The Internet is divided into routing domains, so that the search for a single host's route becomes a search for the hosts in the routing domain, which greatly reduces the size of the routing table and speeds up the search. The search for a routing domain generally distinguishes different routing domains by a routing domain identifier. The classification of the routing domain identifier is generally through the classless inter-domain routing method.

Classless Inter-Domain Routing (CIDR) is a method for classifying IP addresses for assigning IP addresses to users and effectively routing IP data packets on the Internet.

In the first decade after the advent of the domain name system, the design of address allocation and routing of IP packets based on classified networks has clearly become less scalable (see RFC 1517). To address this, the Internet Engineering Working Group released a new series of standards in 1993RFC 1518 and RFC 1519to define new methods for allocating IP address blocks and routing IPv4 packets.

An IP address consists of two parts: the prefix that identifies the network and the host address that follows it within the network. In the previous classification network, the allocation of IP addresses separated the 32 bits of the IP address by 8 bits. This makes the prefix must be 8, 16 or 24 bits. Therefore, the smallest address block that can be allocated is 256 (24-bit prefix, 8-bit host address, 28 = 256) addresses, which is too few for most enterprises. The larger address block contains 65536 (16-bit prefix, 16-bit host, 216 = 65536) addresses, which is too much for large companies. This leads to inadequate use of IP addresses and inconvenience in routing, because a large number of small networks (class C networks) that require separate routing are difficult to perform aggregated routing because they are geographically separated, so a lot of routing equipment is added burden.

Classless interdomain routing is based on variable length subnet masks (VLSM) to assign prefixes of any length. There is a description of variable-length subnet masks in RFC 950 (1985). CIDR includes:

Variable length subnet mask technology specifying prefixes of any length. CIDR-compliant addresses have a suffix indicating the number of digits in the prefix, for example: 192.168.0.0/16. This makes the use of increasingly scarce IPv4 addresses more efficient.

Aggregating multiple consecutive prefixes into a supernet, and in the Internet, whenever possible, it appears as an aggregated network, so the number of entries in the routing table can be reduced overall. Aggregation eliminates the need for the Internet's routing table to be divided into multiple levels and reverses the process of "dividing subnets" through VLSM.

The process of managing the allocation of IP addresses based on the actual needs and short-term expected needs of an organization, rather than oversized or undersized address blocks as defined in the classification network. Because IPv4 CIDR is also used in IPv6 to indicate the prefix length with a suffix, the classification in IPv4 is no longer used in IPv6.

Routing refers to the network-wide process that determines the end-to-end path when a packet goes from source to destination ^[2] . Routing works at the third layer of the OSI reference model, the packet forwarding device at the network layer. Routers implement network interconnection by forwarding data packets. Although routers can support multiple protocols (such as TCP / IP, IPX / SPX, AppleTalk, etc.), most routers in China run the TCP / IP protocol. A router usually connects two or more logical ports identified by an IP subnet or point-to-point protocol, and has at least one physical port. The router determines the output port and next hop address based on the network layer address in the received data packet and the routing table maintained internally by the router, and rewrites the link layer data packet header to forward the data packet. Routers dynamically maintain routing tables to reflect the current network topology, and maintain routing tables by exchanging routing and link information with other routers on the network.