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Sub-addressing
Address checking
Destination address
check
Source address check
38
Network Terminal Number (NTN)
Network addresses within a DNIC (a particular data network provider within a
particular country) are called Network Terminal Numbers (NTNs). NTNs iden-
tify a DTE/DCE interface and are assigned by the data network provider. NTNs
can be of variable length up to 10 digits, depending on the network provider.
National Number (NN)
Where, for example, an integrated numbering scheme exists within a country and
different network providers are identified within the national numbering scheme,
National Numbers (NNs) can be used instead of Network Terminal Numbers
(NTNs). As National Numbers already identify a data network provider within
the country, the International Data Number therefore consists of the Data County
Code (DCC) plus the National Number (NN).
Therefore, a International Data Number
This can consist of any remaining digits of the 14 digit X.121 number, and in
some countries it can be used for sub-addressing within the International Data
Number, for addressing a number of devices/nodes located at a single Interna-
tional Data Number. This can be used for example, for addressing different links
in the router.
To ensure a one-to-one relationship between end devices, a unique number is as-
signed to each link across the PSDN. Incoming calls are only accepted if the local
and remote addresses match, as shown in the table below:
Addresses Must Match for a Call to be Accepted
Calling Device
Local Address
Remote Address
The router only accepts an incoming call when the
the
of an X.25 link in the router.
Local Address
If a
is not defined for an X.25 link, all incoming addresses are
Local Address
accepted by the link (the address check is not performed).
Source address checking adds security to the WAN link—the router only accepts
an incoming call when the source address matches the
figured for the X.25 link.
DNIC NTN or DCC NN.
Receiving Device
Remote Address
Local Address
Remote Address
Remote Address
X.25 Services
X.25 Addressing
matches
con-
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