Editing IS-IS (section)

== IS-IS addressing and NET ==
From regular TCP/IP world we are used to know that each Layer 3 interface (including loopback) has its own IPv4 or IPv6 address. The most important point is that loopback interface always stays up (unless deleted) compared to physical or logical interfaces.

Therefore, ISO chose a different approach - instead of assigning layer 3 address to each interface, single address is assigned to loopback interface, while other interfaces are considered as unnumbered. This single address is called NET (Network Entity Title).

On a single intermediate system there can be up to 3 NET addresses. This is useful during migration from one area to another.

NET consists of '''Area''', '''System ID''' and '''NSEL'''. '''Area''' itself consists of '''AFI''' (Address Family Identifier) and '''Area ID'''.

Area can have variable length of 1 - 13 bytes, System ID is 6 bytes and NSEL - 1 byte.

Let's check on an example NET of 49.0100.1921.6821.1138.00. Here,

* '''49''' is '''AFI''', and in case of 49 it means "private address space", similar to RFC1918 for IPv4.,
* '''0100''' is '''Area ID''',
* '''49.0100''' is '''Area''',
* '''1921.6821.1138''' is '''System ID''',
* '''00''' is '''NSEL''', which '''must''' '''be zero'''. If not zero, then no IS-IS adjacency is formed.