A home or café router has one public address and gives its clients private ones, so it rewrites every packet that crosses it. On the way out, it rewrites the source. On the way back, it rewrites the destination. The NAT table, in the pane on the right, is what drives both. Step runs the clock to the next event, through …

TCP numbers every byte it sends, and every ACK names the next byte it expects, so each acknowledgment can be traced back to the segments it covers. Host A sends from left to right, Host B from right to left. Step plays six scenes, one after another: the handshake, data and close; a lost segment and its retransmission; …

Watch a packet travel from host to server. How to read it Each tower is a device, and each square is a layer of the TCP/IP stack (4 layers, not OSI's 7): Physical, Link, Internet, and Transport — applications live above the stack (the dashed boxes) and use it through sockets: 4-square tower — a host: the full stack up …

For a random IPv4 address + mask (e.g. 178.178.178.178/24), answer three things: Class — is the address Class A, B, or C? Scope — is the mask the class's default, a subnet (longer than default), or a supernet (shorter than default — aggregating multiple classful networks, e.g. 193.15.4.7/16 is one address in a block …

What is DNS Informally, we could say that DNS (Domain Name System) allows users to use names instead of IP addresses. The initial thinking was that names are easier to remember and work with, so users will find it easier to use them. In fact, DNS is part of the network itself. For example, you could run a single …