How the Internet Moves Data

Domains are easier to understand when you can separate names, addresses, connections, and application requests.

A Postal Analogy with Limits

A domain name resembles a contact name, an IP address resembles a routable address, a port resembles a department, and HTTP resembles the format of a request. The analogy is useful, but the internet is not one central postal system. Many independent networks exchange traffic using shared protocols.

The Main Layers

Application:  HTTP, DNS, SMTP
Transport:    TCP, UDP, QUIC
Network:      IPv4, IPv6
Link:         Ethernet, Wi-Fi, mobile network

You do not need to memorize a complete networking model. You do need to identify which layer is failing.

IP Addresses

An IP address identifies a network interface or routing destination.

IPv4 example:

192.0.2.10

IPv6 example:

2001:db8::10

The examples are documentation addresses. Public websites require publicly routable addresses or an intentionally configured proxy or tunnel architecture.

Public and private addresses

Common private IPv4 ranges include:

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

A DNS record pointing public users to a private address normally cannot reach a home or office server across the internet.

Ports

A server can provide multiple services at one address because each connection targets a port.

Port	Common use
22	SSH administration
53	DNS
80	HTTP
443	HTTPS

A listening application, host firewall, network firewall, and upstream network policy can each affect whether a port is reachable.

TCP and UDP

TCP provides an ordered connection and is commonly used by HTTP/1.1, HTTP/2, SSH, and DNS when needed. UDP carries independent datagrams and is commonly used for many DNS queries and QUIC-based HTTP/3.

DNS must not be described as UDP-only. Authoritative DNS needs correct TCP behavior as well.

Clients and Servers

A client begins a request. A server listens and responds.

When you run:

curl https://example.dpdns.org

curl is the client. The machine accepting the HTTPS connection is the server. A reverse proxy may accept the public connection and then become a client of an application behind it.

Routing and Latency

Routers forward packets between networks. The number of network steps and the quality of the path influence latency, but geographic distance is not the only factor.

Test reachability without assuming that an ICMP response proves the website works:

ping -c 4 192.0.2.10

Some networks block ping while permitting web traffic. Use protocol-specific checks for a final conclusion.

Network Address Translation

Home routers often translate many private devices to one public address. Hosting a public service behind such a router may require port forwarding, stable addressing, firewall rules, and an internet connection that permits inbound traffic.

Carrier-grade NAT can prevent ordinary inbound forwarding even when the router shows an address. Confirm the actual network design before choosing home hosting.

A Website Request, Layer by Layer

User enters URL
-> DNS returns an address
-> Routing reaches the network
-> TCP or QUIC reaches a port
-> TLS authenticates and encrypts
-> HTTP requests a hostname and path
-> Web server returns content

Guided Observation Lab

Run these commands against a public site you are permitted to query:

dig A example.com
curl -I https://example.com

Record:

• The returned IP address
• The HTTP status
• Whether a redirect occurred
• The final protocol shown by the client

Do not scan ports or test systems beyond ordinary public DNS and HTTP requests without authorization.

Review Questions

1. Why can correct DNS still lead to a connection timeout?
2. Why does a public A record usually not point to 192.168.1.10?
3. Which layer decides what /about.html means?
4. Why is port 443 not sufficient proof that a valid HTTPS site exists?

Continue to Terminal and File Basics.