Localhost

Some IP addresses are reserved for special purposes. If you have changed your router settings or Wi-Fi password, you may have opened an address such as 192.168.0.1 or 192.168.1.1.

Addresses in the 192.168.x.x range are private IP addresses used inside networks such as homes and offices. They do not uniquely identify a device across the entire public internet. Two different homes can each have a router at 192.168.0.1 without conflict.

Routers do not always use 192.168.x.x. Private ranges also include 10.x.x.x and addresses from 172.16.x.x through 172.31.x.x. The important point is that private addresses identify devices within the local network you are connected to.

127.0.0.1

127.0.0.1 is another special address: an IPv4 loopback address. It points back to the computer you are using. When you send a request there, it returns to your own machine rather than going to another computer.

This lets programs on one computer communicate with each other. The name localhost commonly resolves to 127.0.0.1, or to the IPv6 loopback address ::1. For the IPv4 examples here, we use 127.0.0.1.

flowchart TB
    subgraph computer[Your computer]
        client[Client program]
        server[Server program :8000]
    end

    client -->|Request to 127.0.0.1:8000| server

    classDef client fill:transparent,stroke:#8eb7c2,stroke-width:2px
    classDef server fill:transparent,stroke:#d0a476,stroke-width:2px
    class client client
    class server server
    style computer fill:transparent

Localhost in web development

Localhost is especially useful during development. Production clients and servers may run on separate computers, but you do not need two machines to develop a web service.

Run both programs on your computer and have the client request an address such as http://localhost:8000. The 8000 is the port number we discussed in IP addresses and ports, and is commonly used by development servers. You can build and inspect client-server communication on a single machine.

Exercises

Decide whether each statement is True or False.

1. An address starting with 192.168.x.x uniquely identifies one computer across the entire internet.
2. 127.0.0.1 is a loopback address pointing to your own computer.
3. Localhost lets client and server programs on one computer communicate.