IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback
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IPv6 Private Addresses Types/Scopes
Now, let's take a bit more of a look at private addresses. In IPv6, these are called local-use addresses, the name conveying clearly what they are for. They are also sometimes called link-layer addresses. Recall that IPv4 private addresses were commonly used when public addresses could not be obtained for all devices, sometimes in combination with technologies like Network Address Translation (NAT). In IPv6, trickery like NAT isn't required. Instead, local-use addresses are intended for communication that is inherently designed only to be sent to local devices. For example, neighbor discovery functions using the IPv6 Neighbor Discovery (ND) protocol employ local-use addresses.
The scope of local addresses is obviously a local network, and not the global scope of public Internet addresses. Local addresses in IPv6 are divided further into two types, reflecting a further division of local scope.
These addresses have the scope of an entire site, or organization. They allow addressing within an organization without need for using a public prefix. Routers will forward datagrams using site-local addresses within the site, but not outside it to the public Internet.
Site-local addresses are differentiated from link-local addresses by having a tenth bit of 1 following the nine starting address bits common to all private IPv6 addresses. Thus, they begin with 1111 1110 11. In hexadecimal, site-local addresses begin with FE and then C to F for the third hex digit. So, these addresses start with FEC, FED, FEE or FEF.
These addresses have a smaller scope than site-local addresses; they refer only to a particular physical link (physical network). Routers will not forward datagrams using link-local addresses at all, not even within the organization; they are only for local communication on a particular physical network segment. They can be used for address configuration or for the ND functions such as address resolution and neighbor discovery.
Link-local addresses are differentiated from site-local addresses by having a tenth bit of 0 following the nine initial address bits common to all private IPv6 addresses: 1111 1110 1. Thus, site-local addresses begin with FE and then 8 to B for the third hex digit. So, these addresses start with FE8, FE9, FEA or FEB.
Note that site-local IPv6 addresses are the equivalent of IPv4 private addresses, since they are routed throughout the organization. The concept of link-local scope is new to IPv6.
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