Every router on the path can read, change or replay a packet. TLS seals one connection after a certificate proves the server’s name, IPsec seals everything between two networks, and a firewall decides who may start a connection. Part 8.
IP picks the next router; the link carries the packet there in a frame addressed to a MAC. ARP finds that address, a switch learns where it lives, and a data center connects many equal paths. Part 6.
Nobody types the route from one network to another. Routers announce prefixes with the path they took, pick by policy before path length, and use ICMP to say what went wrong. Part 5.
A router reads one address, takes the longest matching row of its table, lowers the TTL and sends the packet on. Subnets, DHCP and NAT decide which addresses it carries. Part 4.
TCP numbers every byte and resends what is not acknowledged, so a network that loses packets still carries one ordered stream. How fast that stream runs over a long path is at most the smaller of two windows divided by the round-trip time. Part 3.
Before an API call returns its first byte, a resolver turns the name into an address, a socket opens on a port, and HTTP sends a request. Every new connection costs round trips that a kept connection and a cache save. Part 2.
Every hop charges four delays, and distance sets a floor that no faster link lowers. A queue grows sharply near full load, a copy runs at its narrowest link, and a traceroute shows which of these you pay. Part 1.