X25 Addressing Format

The X.25 addressing format is very similar to PSTN. Whenever we talk about the Network User Address (NUA), we mean its internationally standard format (X.121 address). An NUA is composed of

The DNIC is creating with the DCC plus the network code of the X.25 network itself in a specific country, resulting is a four-digit international code.

For example, the DNIC for Italy, ITAPAC X.25 network is 2222:

222 DCC for Italy + 2, which is the network country code for ITAPAC

The (local) NUA begins with the NCC and is then composed of the so-called area code and the network port address (NPA). The NUA standard is 12 digits maximum, even if the average is from 6 to 10 digits, depending on the country and X.25 network size. For example, an NUA might be 21122878 (an old X.25 address from the Politechnic of Turin, Italy) where:

• 11 is for the (PSTN) area code for the town of Turin.

The full X.121 address for this host would then be

222 2 11 22 878 By dissecting it, you obtain the following logic:

| | |__________2: ITAPAC Network (since more networks exist)

| Reading it both externally and locally:

0 222 2 11 22 878 from other networks;

21122878 from Italy/ITAPAC.

This means that if a customer asks you to perform a penetration test on an X.25 address, the first thing to apply is the X.121 address analysis to determine:

• Country where host is located

• If the address is correct for legal authorization

• If the address is working

• The average cost for the X.25 calls needed by the X.25 security testing service you are going to supply

For example, if a customer supplies these NUAs for testing:

• 0311021210126

• 0280221229

• 02624301119090

your analysis should match the following:

DNIC (4)






(USA, SprintNet, NYC)




(Cyprus, CytaPac, Limassol)




(Germany, DATEX-P, Berlin)

More detailed information on X.25 addressing, X.25 hacking and defense techniques, and general tips related to the X.25 world may be found in the following presentations that you can find online:

• Hack in the Box 2005, Kuala Lumpur: X.25 (in)security at http://www .packetstormsecurity.org/hitb05/BT-Raoul-Chiesa-X25-Security.pdf

• Hack in the Box 2007, Dubai: X.25 in the Arab World at http://conference.hitb.org/ Mtbsecconf2007dubai/materials/D2%20-%20Raoul%20CMesa%20-%20X25%20netw orks%20in%20the%20Amb%20World.pdf

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