Subnetting, NAT & Routing

What is the primary purpose of subnetting?

To limit Ethernet broadcast traffic and logically segment hosts, typically by building/department in IPv4 networks.

How many usable addresses are in a 152.78.70.0/24 IPv4 subnet?

254 total (256 minus network and broadcast addresses), with typically 253 left after assigning one to the router.

What are the three RFC 1918 private address ranges?

10.0.0.0/8 (16M), 172.16.0.0/12 (1M), 192.168.0.0/16 (65K).

How does NAT enable private networks to access the internet?

By sharing a single global IPv4 address among multiple hosts, translating private addresses/ports (NAPT).

What is CGNAT and why is it used?

Carrier-Grade NAT (RFC6598) shares public IPs among customers via 100.64.0.0/10 addresses, mitigating IPv4 exhaustion.

How does a netmask determine network boundaries?

Specifies prefix length (e.g., /24 = first 24 bits are network); non-matching addresses require router forwarding.

What are the two routing options for a host's outbound packet?

Direct delivery to local subnet hosts or forwarding to a router (default route) for external destinations.

What determines priority in routing table entries?

Longest prefix match first, then lowest metric for equal-length prefixes.

How does prefix aggregation reduce routing table sizes?

Combines adjacent subnets (e.g., 192.168.10.0/24 + 11.0/24 → 10.0/23) to minimize advertised routes.

What is an Autonomous System (AS)?

A network/group with unified routing policy, assigned an ASN (e.g., JISC AS 786), categorized as multihomed/transit/stub.

Compare distance-vector and link-state IGP protocols.

Distance-vector (RIP): shares best hops with neighbors only. Link-state (OSPF/IS-IS): floods full topology for Dijkstra-based paths.

What are RIP's key limitations?

15-hop max, slow 30s updates, unacknowledged UDP, MD5 auth (broken), no topology awareness causing "count-to-infinity."

How does OSPF build routing tables?

Discovers neighbors → floods link-state packets → computes shortest paths via Dijkstra’s algorithm → populates tables.

What makes BGP the de facto EGP protocol?

Shares AS-path attributes (not just metrics), enables policy-based routing between ASes via TCP peering (port 179).

Why is BGP slow to converge?

Default 60s keepalive/180s holddown timers prevent flapping; updates are incremental after initial full table exchange.

How does traceroute map network paths?

Sends packets with increasing TTL, records ICMP Time Exceeded responses (UDP on Linux/macOS, ICMP on Windows).

Why can’t traceroute detect asymmetric routing?

Paths may change mid-test; return routes often differ from outgoing paths (latency ≠ RTT/2).

What are common routing "gotchas"?

Dynamic path changes, asymmetric routes, variable link speeds, and transient congestion/errors.

How do IPv6 routing tables differ from IPv4?

Similar principles but larger addresses, no NAT, and simplified header processing for efficiency.