IS-IS (Intermediate System to Intermediate System) Protocol uses NSAP (Network Service Access Point) addresses instead of IP addresses. NSAP addresses consist of an area address, system ID, and N-selector. The area address contains an AFI, IDI, and HODSP. NSAP addresses can also be divided into an interdomain part containing an AFI and IDI, and a domain-specific part containing HODSP, system ID, and NSEL. IS-IS uses different multicast addresses for L1 and L2 updates depending on the network type.

1. IS-IS ProtocolAddresses
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2.  IS-IS (Intermediate System to Intermediate
System) Protocol was developed as a OSI network
protocol, not TCP/IP. So, normally IS-IS Protocol
not uses IP address, but it uses NET (Network
EntityTitle) addresses.
 In IS-IS Protocol, NSAP (Network Service Access
Point) is used as network layer address. These
addresses are assigned per node, not per interface.
 And SNPA (Subnetwork Point of Attachment) is
the Layer 2 addresses for IS-IS Protocol.

3. NSAP consist
of three parts.
These are:
- Area-address
-System ID
- N-Selector
Area address also has some parts inside it.These parts are AFI,IDI
and HODSP.

4. In previous slide, we have checked an example ISIS
address,
49.0115.125a.ab44.1234.11111.0123.abcd.aaaa.00.
AS you can see, 49 is the AFI. 0115.125a.ab44.1234 is
the IDI part. For Area Address, last part is HODSP
and its value is 1111.
System ID part of this example address is
0123.abcd.aaaa and the last part SEL part is 00.

5. We can also divide
the NSAP address
into two parts as
(1) Inter Domain
Part and (2)Domain
Specific Part.
1. Inter Domain Part consists of AFI (Authority and Format
ID) and IDI (Inter Domain ID).
2. Domain Specific Part consists of High Order Domain
Specific Part, System ID and NSEL part. Here, High
Order DSP is the subnet, System ID is the identifier of the
device in area and the NSEL is the service identifier of
NSAP address. When NSEL is equal to “00”, it is
called Network Entity Title (NET) and it shows the device
itself.
 NETs are 8 to 20 bytes long. But in general, they are 10
bytes long.
 System ID can be thought like OSPF Router ID. NSEL is like
port/socket inTCP / IP.

6.  You need to request an official NSAP prefix if you will
use CLNS routing. Even if you will need only IP
routing when using IS-IS Protocol, you always needs
to use NSAP addresses, but this time you can add
the private address space AFI 49.
 AFI 49 is used as local addresses. This addresses are
not advertised outside of CLNS network.
 In IS-IS ProtocolL2 Multicast Addresses are also used.
And LSPs are sent to these addresses. In a broadcast
network, L1 updates are sent
to 0180:C200:0014 and L2 updates are sent
to0180:C200:0015.
 In Point-to-Point networks 0900:2b00:0005 address is
used as L2 address.

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