Crossing the world

A packet leaves your laptop addressed to a server 6,000 km away. Your router does not have a map of the world. It has a table with a few dozen entries, most of them starting with "if it is not mine, send it to the provider".

The journey works because every router makes one small decision, repeatedly, and because 80,000 independent networks have agreed to forward each other's traffic.

5.1Three columns and one rule

A routing table entry has a destination prefix, a next hop, and an interface. The lookup rule is longest prefix match: among all entries that match, the most specific one wins — before any metric, cost or preference.

5.2What a hop actually does

The router receives a frame, discards the link header, reads the destination IP, looks it up, finds the next hop, resolves the next hop's hardware address, builds a new frame, and transmits. TTL decreases by one at each hop; at zero the packet is dropped and an ICMP "time exceeded" comes back.

That TTL behavior is the entire mechanism behind traceroute: send a packet with TTL 1, the first router drops it and reports itself; send with TTL 2, the second router reports; repeat. The path you see is a list of routers that were willing to talk.

traceroute example.com      # macOS/Linux; use tracert on Windows
mtr -rw example.com         # repeated probes, with loss per hop
ip route get 203.0.113.7    # which route this host would use

A star (* * *) in traceroute is not a broken hop. It is a router configured not to answer — common, and usually harmless. Loss that appears at one hop and continues to every later hop is real; loss at one hop that does not continue is the router rate-limiting its own error replies.

5.3Providers, autonomous systems, and trust

Inside one organization, routers share routes with an interior protocol — OSPF or IS-IS — which floods a map of the network and computes shortest paths. Between organizations, they use BGP, and BGP works on policy rather than distance.

An autonomous system is a network under one administration with a number (an ASN). It announces the address ranges it can reach, and each neighbour decides whether to believe it, and whether to prefer it over another path. The path itself is carried as a list of AS numbers, so a router can spot its own number in a route and reject the loop.

5.4The leftover route

Routers prefer the most trustworthy source when two protocols know the same prefix: connected first, then static, then learned protocols. On most vendors a static route has administrative distance 1 and OSPF has 110, so a static route wins silently.

That is how a decommissioned link keeps taking traffic: a static route added for a migration stays in the configuration for years, and the only evidence is a route that does not match the diagram. When traffic goes the wrong way and every protocol is "up", read the routing table before the topology.