Why this matters
Every device on a network needs an address so data knows where to go, and every device must agree on the rules for talking. Without addresses and shared protocols, the internet would be a crowd of computers shouting in different languages with no way to find each other.
The idea
An IP address is a unique number assigned to a device, just like a street address. The common
form, IPv4, uses 32 bits written as four blocks separated by dots, each block from 0 to
255 (for example 192.168.0.1). Because the internet grew so fast, IPv4 addresses have almost
run out, so we are moving to IPv6, which uses 128 bits for vastly more addresses.
Addresses come in two kinds: a global IP address is used out on the internet, while a
private IP address is used only inside a local network. Numbers are hard for people to
remember, so a domain name (like example.co.jp) stands in for the number. The DNS system,
run by DNS servers, translates between domain names and IP addresses.
Talking requires shared rules called protocols. Data is split into packets, each carrying a header with destination info. The key protocols are TCP (splits data into packets, reorders them, and re-requests lost ones, so it's reliable), UDP (fast and real-time with no guarantees, good for calls and streaming), and IP (assigns addresses to route packets). Together these form TCP/IP, organized into four layers:
| Layer | Name | Job | Example |
|---|---|---|---|
| 4 | Application | App-to-app communication | HTTP, SMTP |
| 3 | Transport | Control, error detection, retransmission | TCP, UDP |
| 2 | Internet | Address allocation and routing | IP |
| 1 | Network interface | Physical connection between devices | Ethernet |
Picture it
flowchart LR D[Domain name: example.co.jp] -->|DNS lookup| N[DNS server] N -->|returns| IPaddr[IP address: 192.168.0.1] IPaddr --> P[Split into packets] P -->|TCP reorders, IP routes| Dest[Destination device]
Worked example
Which of these is a valid IPv4 address: 10.123.45.67 or kantei.go.jp? IPv4 is four numbers from
0 to 255 separated by dots, so 10.123.45.67 is the address; kantei.go.jp is a domain name.
Now, if you make a voice call, should the app use TCP or UDP? A call must arrive in real time, and a
tiny gap is better than a long wait for a re-sent packet, so it uses UDP. Downloading a file,
where every byte must be correct, uses TCP.
Your turn
Try the practice questions: match each protocol to its job, and sort the TCP/IP examples into the right layer.
Recap
- An IP address identifies a device; IPv4 is 32 bits (0–255 blocks), IPv6 is 128 bits.
- DNS translates domain names to IP addresses and back.
- TCP is reliable, UDP is fast, and IP routes; together they are TCP/IP in four layers.