The Internet and DNS

The Internet is a global network of networks — millions of individual networks (home networks, corporate networks, data centres, university networks) interconnected by routers. No single organisation owns or controls the Internet.

Key properties:

• Packet-switched — data is broken into packets and each packet is routed independently
• Decentralised — there is no central point; packets can take many different paths
• Interoperable — different physical networks (fibre, copper, WiFi, satellite) all use the same TCP/IP protocol

Packet switching is the method by which data travels across the Internet.

How it works:

1. The sender's device breaks the message into fixed-size packets
2. Each packet is sent independently through the network
3. Packets may take different routes and arrive out of order
4. The receiver reassembles packets in the correct order using sequence numbers

Packet structure

Each packet contains:

Part	Content
Header	Source IP address, destination IP address, sequence number, protocol, TTL (time to live)
Payload	The actual data (fragment of the message)

Advantages of packet switching:

• Network resources are shared efficiently — links are occupied only when packets are being sent
• If one route is congested or fails, packets can be rerouted automatically
• No dedicated circuit needed between sender and receiver

A router is a device that forwards packets between different networks. It reads the destination IP address in each packet's header, consults its routing table, and forwards the packet toward the destination.

How routing works:

1. Packet arrives at router
2. Router looks up the destination IP in its routing table
3. Router forwards the packet to the next hop (next router or final destination)
4. Process repeats at each router until the packet reaches its destination

Gateway: a router that connects different network types (e.g. a home router acts as a gateway between the home network and the ISP's network, bridging different addressing schemes).

IP address: a numerical label assigned to every device on a network. Used by routers to identify the source and destination of packets.

Domain name: a human-readable name that maps to an IP address (e.g. www.example.co.uk). Humans remember names; routers use IP addresses.

FQDN (Fully Qualified Domain Name): the complete domain name of a specific host, including all hierarchical levels.

Reading www.example.co.uk from left to right, each label is one level of the hierarchy:

Part	Level
www	subdomain / host
example	registered domain
.co	second-level domain
.uk	top-level domain (TLD)

URL (Uniform Resource Locator): the complete address of a specific resource, including protocol, domain, and path:

In https://www.example.co.uk/products/item.html, the three components are:

Component	Part
https	protocol (HTTPS)
www.example.co.uk	FQDN
/products/item.html	path to the resource

DNS translates domain names into IP addresses. It is a hierarchical distributed database — no single server holds all records.

DNS resolution process (simplified):

1. User types www.example.co.uk in their browser
2. Device checks its local DNS cache — if found, use cached IP
3. If not cached, query sent to the local DNS resolver (usually the ISP's DNS server)
4. Local resolver queries the root DNS server → directed to the .uk TLD server
5. TLD server → directed to example.co.uk's authoritative name server
6. Authoritative server returns the IP address for www.example.co.uk
7. IP address returned to the device; cached for future use
8. Browser connects to the IP address

Why hierarchical? No single server could store all domain name records for the entire Internet (hundreds of millions of domains). Distributing the database across thousands of servers makes it scalable and resilient.

Internet registries are organisations that manage the allocation and registration of Internet resources.

Registry	Role
IANA (Internet Assigned Numbers Authority)	Top-level allocation of IP address blocks and TLD management
Regional Internet Registries (e.g. RIPE NCC for Europe)	Allocate IP addresses to ISPs and organisations within a region
Domain registrars	Allow individuals/organisations to register domain names under TLDs

Internet registries maintain authoritative databases of who has been assigned which IP addresses and domain names, preventing conflicts.

1. Confusing IP address and domain name

An IP address (93.184.216.34) is what routers use. A domain name (www.example.com) is what humans use. DNS translates between them. They are not interchangeable terms.

2. Stating all packets take the same route

In packet switching, different packets from the same message may travel different routes through the Internet. They are reassembled in order at the destination using sequence numbers.

3. Confusing URL and FQDN

A FQDN identifies a specific host (www.example.co.uk). A URL identifies a specific resource, including the protocol and path (https://www.example.co.uk/page.html). A FQDN is a component of a URL.

4. Claiming DNS is a single central server

DNS is a hierarchical distributed system — root servers, TLD servers, authoritative servers, and local resolvers each handle part of the resolution process. There is no single DNS server; the system is designed to be resilient and scalable.