IS-IS NET can be regarded as a special NSAP address, which means that the transmission protocol is IP. That is, the SEL flag of the NET address is fixed to (___).

Question

Options

• A3
• B
• C
• D

• A
  84% (21)
• B
  4% (1)
• C
  4% (1)
• D
  8% (2)

Explanation

Option A is correct because in IS-IS, the NET (Network Entity Title) is a special NSAP address where the NSEL (N-Selector, or SEL) byte is always set to 0x00. This zero value signals that the address identifies the network layer entity itself - the router - rather than a specific transport-layer protocol or service endpoint. When IS-IS is used as an IP routing protocol, this convention still holds: the SEL remains 0x00 to distinguish the NET from a full NSAP, which would have a non-zero SEL pointing to an upper-layer protocol. The distractor options represent non-zero SEL values, which would imply the address belongs to a specific transport service, contradicting the definition of a NET. As a memory aid: think "NET = no upper layer = zero selector" - the SEL byte is zero precisely because the NET addresses the network entity, not any layer above it.

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