Start with the conversation
An address identifies an interface; a port helps identify a transport endpoint. A well-known port is a convention, not proof of what a service actually runs.
- HTTPS · TCP 443; HTTP/3 uses QUIC over UDP, commonly on 443.
- DNS · UDP or TCP 53; encrypted DNS uses other transports.
- SSH · TCP 22 for encrypted remote access.
- SMTP · TCP 25 for mail transfer; submission commonly uses 587 or 465.
- DHCPv4 · UDP 67/68 for address configuration.
- NTP · UDP 123 for time synchronisation.
TCP, UDP and QUIC
TCP provides an ordered byte stream with retransmission and congestion control. UDP provides datagrams without built-in delivery or ordering guarantees. Applications decide what reliability they need.
- TCP does not preserve application message boundaries.
- UDP is useful when message boundaries or timely delivery matter.
- QUIC adds secure transport, independent streams and congestion control over UDP.
- A TCP handshake establishes transport state; it does not authenticate the application.
IPv4 and IPv6
IP routes packets across network boundaries. IPv4 uses 32-bit addresses; IPv6 uses 128-bit addresses. Neither guarantees delivery on its own.
- ARP maps IPv4 addresses to local link-layer addresses.
- IPv6 uses ICMPv6 neighbour discovery instead of ARP.
- ICMP reports conditions such as an unreachable destination or a packet that is too large.
- Routers forward using the most specific matching destination prefix.