The Internet: protocols, addressing and packet switching

Internet versus World Wide Web

The Internet is the global network infrastructure; the Web is one service running on it, alongside email and file transfer. Getting this distinction right is a standard one-mark question.

The TCP/IP stack

Four layers. Application (HTTP, FTP, SMTP) speaks to the program. Transport (TCP) splits data into packets, numbers them, acknowledges and retransmits. Internet (IP) adds source and destination addresses and routes. Link puts the bits on the physical medium. Each layer adds a header on the way down and the receiver strips them on the way up — layering means one layer can change without rewriting the others.

Packet switching

Data is split into packets, each carrying a header (source, destination, sequence number) and a trailer (error check). Packets may take different routes and arrive out of order; the receiver reorders by sequence number and requests missing packets. Compared with circuit switching, packet switching uses the network efficiently and survives a broken link, but adds delay and cannot guarantee ordering.

Addressing

MAC is a fixed hardware address used within a local network; IP is a logical address that changes with location and is used to route between networks. IPv4 is $32$ bits and has run out; IPv6 is $128$ bits. Public addresses are routable on the Internet; private ones are not, which is why NAT exists.

DNS

DNS turns a domain name into an IP address. The resolver asks a root server, then a TLD server, then the authoritative name server for the domain; results are cached to avoid repeating the walk. Without DNS the Web still works — but only if you type $142.250.…$ instead of a name.