IP
The internet's network protocol: best-effort datagrams and hierarchical addresses.
Level: beginner
What is it?
Internet Protocol (IP) delivers packets across interconnected networks using logical addresses and routing. IP is connectionless and best-effort: it does not promise that each packet arrives, arrives once, or arrives in order.
Why does it matter?
IP provides a common layer across different link technologies. Reliability, ordering, congestion behavior, and application-level semantics are supplied by other protocols when needed.
Mental model
Think of IP as a postal addressing and forwarding system. Each router forwards a packet closer to the destination based on a routing table; intermediate routers do not establish a reliable end-to-end conversation by themselves.
How it works
- IPv4 uses 32-bit addresses and IPv6 uses 128-bit addresses.
- Routers forward based on destination prefixes and next-hop decisions.
- TTL in IPv4 (Hop Limit in IPv6) limits packets from circulating indefinitely.
- IP may fragment or require packetization choices depending on version and path MTU; IPv6 routers do not fragment packets in transit.
- TCP, UDP, and application logic provide behavior beyond IP's best-effort delivery contract.
Steps
- A host creates an IP packet for a destination address.
- It checks whether the destination is local or needs a configured gateway.
- The packet is wrapped in the local link's frame and sent to the next hop.
- Each router consults its forwarding table and transmits the packet on a new link until it reaches the destination network.
Real-world usage
- Reading packet captures and routing tables.
- Explaining how traffic crosses multiple networks.
- Distinguishing link-layer delivery from end-to-end IP routing.
Common mistakes
- Assuming IP guarantees delivery or in-order arrival.
- Assuming the destination IP is necessarily a device's hardware address.
- Treating all addresses in the same subnet prefix as directly reachable without considering routing and policy.
Interview questions
What service does IP provide?
Best-effort packet delivery across networks using logical addresses and routing, without an end-to-end delivery guarantee.
Why is a default gateway needed?
It provides the next hop for destinations not covered by the host's directly connected routes.