16.4 How the Internet Works: Protocols, IP Addressing, DNS, URLs, and Reliability

What this competency asks

ETS asks you to be familiar with how Internet and Web protocols work:

1. Describe the purpose of protocols and identify common Internet and Web protocols.
2. Compare IPv4 and IPv6.
3. Identify and describe the basic parts of a URL (for example, protocol, subdomain, domain name, port, and path).
4. Describe the hierarchical structure of names in the Domain Name System (DNS).
5. Describe the purpose and function of IP addressing.
6. Identify how Internet protocols address reliability, redundancy, and error handling.

Why protocols?

A protocol is a set of rules for how data are formatted, sent, received, and acknowledged. Because the Internet's protocols are open standards, a phone from one company can load a web page from a server built by another over equipment made by a third. Protocols are organized in layers (Section 16.2).

Ports identify which program on a device should receive the data. The IP address gets data to the right computer, and the port gets it to the right application.

IP addressing

An IP address identifies a device's network interface and its location in the network, so routers can forward packets toward it, much as a street address does for mail.

• DHCP usually assigns addresses automatically (dynamic). Servers often keep static addresses.
• Private addresses, such as 192.168.x.x, 10.x.x.x, and 172.16.0.0–172.31.255.255, are used inside local networks and are not routed on the public Internet. A router's NAT lets many private devices share one public address.

IPv4 vs. IPv6

(The addresses 192.0.2.x and 2001:db8:: are reserved for documentation examples.)

Parts of a URL

A URL (uniform resource locator) identifies a resource on the Web.

https://www.example.edu:8443/classes/cs101/index.html?week=3#quiz

Watch for look-alike domains in phishing links. In https://example.edu.login-check.com/, the actual registered domain is login-check.com, and example.edu is only a subdomain of it.

The Domain Name System

People use names, and routers use numbers. DNS translates between them. Names are hierarchical, read from right to left:

.                       (root)
└── edu                 (top-level domain)
    └── example         (second-level domain: example.edu)
        └── www         (subdomain / host: www.example.edu)

Resolving www.example.edu:

1. The device checks its own cache, then asks its configured recursive resolver, typically run by the ISP, school, or a public DNS service.
2. If the answer is not cached, the resolver asks a root server, which refers it to the .edu TLD servers.
3. A .edu server refers it to the authoritative servers for example.edu.
4. The authoritative server returns the IP address (an A record for IPv4 or an AAAA record for IPv6).
5. The resolver caches the answer for its time-to-live (TTL) and returns it to the device.

DNS is distributed: no single server holds every name, and each zone is served by multiple servers, which gives both scale and redundancy.

Reliability, redundancy, and error handling

TCP or UDP? TCP establishes a connection with a three-way handshake (SYN, SYN-ACK, ACK) and guarantees ordered, complete delivery, so web pages, email, and file transfers use it. UDP skips the handshake and guarantees, trading reliability for speed, so live video, voice calls, games, and most DNS lookups use it. For them, a late packet is useless anyway.