How Packets Find Their Way

Every packet hits the routing table. One row decides where it goes. No match means dropped. IP addressing, routing, ICMP, and forwarding.

Labs in this module

IP Addresses and Subnets - One card, one MAC, two IP addresses. CIDR is the ruler, the kernel picks its identity per destination, and your own address is just a /32 ro
The Routing Table - The kernel checks this table for every single packet. ip route get shows the exact decision for one destination.
ICMP and Ping - ICMP is how IP reports its own failures. Send echo requests, catch them on the wire, dissect every field, break a ping on purpose, and map a
IP Forwarding - Flip one bit and your Linux box becomes a router. /proc/sys/net/ipv4/ip_forward controls everything.
Packet Filtering - A firewall inspects Layer 3 and Layer 4 headers and makes a decision per packet. nftables is how Linux does it.