One program on one computer is trying to send bytes to a program on another computer. Everything in these notes is how that arrow works, or fails.
The picture
Left: a process on your laptop. Right: a process on another machine. The arrow is packets, each carrying to this IP, this port. Layer names later have somewhere to sit if you keep this picture.
Four facts
If a connect fails and you cannot point at one of these four, stop and name them on paper before you capture packets.
- Address. Which machine. Almost always an IP.
- Door. Which process. That is a port.
- Path. What sits between: NAT, a filter, a hop.
- Talk. TCP (an agreement) or UDP (a throw).
A TCP connection is source IP, source port, dest IP, dest port. Dest port 443 is shared. The conversation is the row.
That is the sample. The PDF walks ping versus 443, NAT, localhost, HTTP last, and the questions I want answered in a room.