IBM SC30-3865-04 - Manual (Page 328)

4700061234CCCC222222222222
The encoded address prefix is a result of the truncation of the NSAP
4700061234
The encoding rules are about all NSAP formats having a fixed length IDI and to any
address prefix ending after the IDP.
Encoding an AFI
An address prefix based entirely on the AFI is encoded only on the 1 octet AFI field.
For example, if an address prefix is needed for all X.121 format addresses (used on
X.25 networks), you would use the X.121 AFI of 37.
Encoding a Variable Length IDI
NSAP addresses that have variable length IDI formats, such as X.121, F.69, E.163,
and E.164, use a more complicated encoding scheme. When variable length IDIs
are encoded as an NSAP, the address is left padded with zeros; however, when the
IDI is encoded as an address prefix, there is no left padding.
For example, you want to route X.25 calls from the U.S. to an X.25 carrier in the
Netherlands. The carrier has a Data Network Identifier Code (NDIC) of 2041. The
encoding of the address prefix would be
372041
An X.25 subscriber having a national telephone number (NTN) of 117010 on this
carrier would have an NSAP of
3700002041117010
Notice that the IDI of the NSAP is left padded with zeros to 14 digits because the
resulting international data number (2041117010) was less than 14 digits.
If, however, you want an address prefix that points only to this one X.25 subscriber,
the encoding would then be the NSAP (3700002041117010), because the prefix
does not end in the IDP.
Default Address Prefixes
A default address prefix is used when you want to originate a default route to all
addresses outside your domain. Default address prefixes are of zero length, so
there is nothing to encode.
Authentication Passwords
To provide a minimum layer of security to the network, OSI provides the option of
authentication passwords. When authentication is enabled, any IS-IS packet that
does not contain the proper password is not accepted by the IS. The authentication
field of the NPDU contains the authentication passwords. There are two types of
authentication passwords, transmit and receive.
A transmit password is added to IS-IS packets transmitted by the IS. A receive
password is a listing of the transmit passwords that the IS accepts. For example,
with authentication enabled, if a transmit password is not added to the packet, or a
Using OSI/DECnet V
298 MRS V3.2 Protocol Config Ref Vol 2