IP

An IPv4 address is 32 bits written as four decimal octets, each 0-255, such as 198.51.100.37. The subnet mask's one-bits mark the network part and its zero-bits the host part; ANDing address and mask gives the network.

Unicast reaches one interface. Limited broadcast, 255.255.255.255, reaches every host on the sender's link and is never routed. A directed broadcast (a network's all-ones host address) reaches every host on that remote network. Multicast, 224.0.0.0-239.255.255.255, reaches joined group members.

The MTU is the largest IP datagram, header included, that a link carries in one frame: 1,500 bytes on Ethernet. An IPv4 router fragments a bigger datagram and only the destination reassembles it; with Don't Fragment set, the router drops it instead.

IPv4's Time to Live is an 8-bit counter each router decrements by at least one; a router that brings it to zero discards the datagram and, for unicast, returns ICMP Time Exceeded to the source, so looping packets cannot circulate forever.

The source assumes the path MTU equals its first link's MTU and sets DF on every datagram. A router that cannot forward one drops it and returns an ICMP error carrying the next hop's MTU; the source lowers its estimate and sends smaller.

An IPv6 address is eight 16-bit hex groups separated by colons. RFC 5952 canonical form drops leading zeros, uses lowercase, and writes :: once, for the longest run of two or more zero groups, the first run on a tie.

IPv6 has unicast (delivered to one interface), multicast (delivered to every member of a group, ff00::/8) and anycast (delivered to the nearest of a set, using unicast-format addresses). Broadcast was dropped; scoped multicast groups such as all-nodes ff02::1 replace it.

IPv6 Neighbor Discovery (RFC 4861) replaces ARP: a host sends an ICMPv6 Neighbor Solicitation to the target's solicited-node multicast address, and the target answers with a unicast Neighbor Advertisement carrying its MAC. IPv6 has no broadcast.

IPv4 and IPv6 are separate protocols with different headers and address sizes, so neither stack can read the other's packets. Coexistence comes from dual stack (run both), tunnelling (carry IPv6 inside IPv4) or translation (NAT64 rewriting headers).

IPv6 widens addresses to 128 bits and moves work off routers: a fixed 40-byte header with extension headers, no header checksum, source-only fragmentation, multicast instead of broadcast, Neighbor Discovery instead of ARP, built-in autoconfiguration, and no NAT in its base design.

The number after the slash is the prefix length: how many leading bits of the 32-bit IPv4 address are network bits. /20 is twenty ones then twelve zeros, 255.255.240.0; counting the ones in 255.255.240.0 gives 20 back.

It is an RFC 3927 IPv4 link-local address: the host obtained no address from DHCP or manual configuration and assigned itself one. It picked a pseudo-random address, confirmed it was free with ARP probes, and can reach only hosts on its own link.

RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0 to 172.31.255.255) and 192.168.0.0/16. Anyone may reuse them without registration, so they are not unique, and routes and packets for them are kept off inter-network links.

An IPv4 /n prefix leaves 32 - n host bits: 2^(32 - n) addresses, minus the all-zeros network and all-ones broadcast addresses. To size a subnet, pick the fewest host bits h whose 2^h - 2 covers the hosts needed.

VLSM (variable-length subnet masking) carves one IPv4 block into subnets with different prefix lengths, each sized to its segment. A single fixed mask must fit the largest segment, which wastes space on small ones and yields too few subnets.

An IPv4 router validates the header, finds the longest matching prefix for the destination address, decrements the TTL, resolves the next hop's link-layer address, sends the packet in a new frame, and then forgets it.

10.1.1.0/24 forwards it. All four entries match 10.1.1.7, and IPv4 forwarding picks the longest matching prefix, so the most specific route wins; the default route 0.0.0.0/0 is used only when nothing longer matches.

Each router must lower the 8-bit IPv4 TTL by at least one; at zero it discards the packet and sends ICMP Time Exceeded to the source, so a looping packet dies after a bounded number of hops.

Routes are IP-layer knowledge that must work over any link, so the next hop is an IP address; the router then resolves it to a link-layer address (ARP on IPv4 Ethernet) and frames the unchanged packet to it.

Requests are routed by lookups of the server's address and replies by lookups of the client's, often in different routers, so paths can differ; a stateful firewall seeing one direction lacks half the connection and typically drops it.

A connected route appears when an interface with an address and prefix comes up; a static route is configured by hand; the default route (0.0.0.0/0, ::/0) matches everything and is used only when nothing more specific matches.

Only routes for the same prefix compete. Among them the router keeps the source with the best administrative distance, an implementation-defined trust ranking where lower wins; a metric ranks routes only within one protocol. With common defaults the static route wins.

A distance-vector router learns only its neighbours' distances to each destination and adds its link cost (Bellman-Ford); a link-state router floods descriptions of its own links, so every router holds the whole topology and runs shortest-path-first itself.

An interior gateway protocol finds best paths inside one autonomous system; an exterior gateway protocol exchanges reachability between autonomous systems. BGP-4 fills that role because it carries AS paths and applies each operator's policy, which a shared shortest-path metric cannot express.

A floating static route is given a worse administrative distance than the dynamic route, so it stays uninstalled until that route is withdrawn. It misses failures that never withdraw the primary, and dead backup paths behind an up interface.