Unicast addresses

There are several forms of unicast address assignment in IPv6. These are:

• Aggragatable global unicast addresses

• Unspecified addresses

• Loopback addresses

• IPv4-based addresses

• Site local addresses

• Link local addresses.

Aggragatable global unicast addresses are used for global communication. The first 3 bits identify the address as an aggregatable global unicast address. The remainder of the bits is sub-divided into fields in order to provide a hierarchical identification of the network providers, networks, sub-networks and end-user devices.

Unspecified addresses can be written as 0:0:0:0:0:0:0:0, or simply '::'. These can be used as source addresses by stations that have not yet been configured with IP addresses. They can never be used as destination addresses. This is similar to 0.0.0.0 in IPv4.

Loopback addresses (0:0:0:0:0:0:0:1) can be used by a node to send a datagram to itself. It is similar to the 127.0.0.1 of IPv4.

An IPv4-based IPv6 address can be constructed out of an existing IPv4 address. This is done by prepending 96 zero bits to a 32-bit IPv4 address. The result is written as 0:0:0:0:0:0:192.100.100.3, or simply ::192.100.100.3.

Site local addresses are partially equivalent of the IPv4 private addresses (i.e. addresses for private use that cannot be used on the Internet). A typical site local address will consist of the relevant prefix, a set of 38 zeros, a sub-net ID, and the interface identifier. Site local addresses cannot be routed in the Internet, but only between two stations on a single site.

Link local addresses are used by stations that are not yet configured with either a provider-based address or a site local address may use link local addresses. Theses are composed of the link local prefix, 1111 1110 10, a set of 0s, and an interface identifier. These addresses can only be used by stations connected to the same local network and packets addressed in this way cannot traverse a router.

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