X.25

• X.121 address format

• Data Network Identification Code (DNIC): 4 digits, first 3 digits specify the country, last digit specifies PSN

• National Terminal Number (NTN): used to specify a DTE on the PSN

Destination DTE

address length

Source DTE

address length

Country

PSN

National terminal

number

DNIC 4 digits

NTN up to 10 digits

4 bits

4 bits

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