Unicasting

Unicasting is an architecture or technique that concentrates on sending information to a single destination, or finding the most appropriate path to a given single destination. It is an alternative to multicasting, broadcasting, and anycasting, which, in more than trivial contexts, are overlays and refinement to the basic unicast process.

Addressing

In media access control addressing at the data link layer (at layer 2), or in Internet protocol version 4 and Internet protocol version 6, there are various address formats that either are unicast or not. A multicast or broadcast address may be a shorthand for a particular group of unicast addresses; that group may all addresses of a given kind, or may represent a dynamic group that given unicast destinations can join or leave.

Depending on the protocol and layer involved, a given address, especially multicast, may combine or separate two functions. The first is that of an identifier which uniquely identifies a place in the network. The second is that of a locator, which gives information about how to reach that place.

Unicast routing

Unicast routing establishes paths, usually not to individual addresses but to the locality where that address resides. If you were driving, basic unicast routing would take you to the street where a given house address exists, and expect that you can find the house once there. Another way to read "basic" is "interior", or a routing domain under the policy control of one administrative authority.

Exterior routing is the equivalent of giving you the information to drive to a particular city, in which interior routing mechanisms will find the street in question. The distinction is not always clear, as unicast routing mechanisms intended for hierarchical networks may have multiple mechanisms, one to get you to a given area, and a second to find particular streets within that area.