Networking · foundation
IP address and network interface
A network interface is an operating-system attachment point for sending and receiving network packets, whether backed by hardware or software. An IP address and prefix assigned to an interface identify an IP endpoint and its on-link network within a particular network configuration.
Why it matters
Connectivity failures often come from the wrong source address, missing route, down interface, or incorrect prefix rather than from the application. Inspecting interfaces and routes locates the problem before higher-level debugging.
Mental model
How to reason about ip address and network interface
The OS chooses a route for each destination, then selects an outgoing interface and source address consistent with that route. An address belongs to a configured network context; it is not an immutable serial number for a physical machine.
Analogy
An interface is a building's doorway and an IP address is a delivery address currently assigned to that doorway. A building can have several doors and addresses, and an address can be reassigned later.
Examples
See the boundary, not just the happy path
Worked example · Inspect configured addresses
ip address showOn Linux, this lists interfaces, their state, and assigned IPv4 or IPv6 prefixes. An interface can have multiple addresses, including temporary or link-local ones with different scopes.
Worked example · Ask which route will be used
ip route get 1.1.1.1The kernel evaluates its routing policy and reports the chosen next hop, interface, and usually source address. This is more reliable than guessing from the interface list alone.
Useful contrast · IP address versus link-layer address
192.0.2.10 is an IP address; 02:42:ac:11:00:02 resembles a MAC addressIP addresses support routing across networks. Link-layer addresses identify interfaces on a local link and serve a different protocol layer and scope.
Common mistakes
Misconceptions to remove early
Assuming a host has exactly one IP address
A system can have several interfaces, IPv4 and IPv6 addresses, virtual interfaces, and multiple addresses per interface. The route can select different source addresses for different destinations.
Reading the prefix as part of the host address
In 192.0.2.10/24, /24 states how many leading bits form the network prefix. It is routing information, not an extra decimal component of the address.
Quick check
Can you predict the result?
1. What determines which local interface sends a packet to a destination?
- • The operating system's routing and policy decision.
- • The alphabetically first interface name.
- • The DNS record's time to live.
2. Why might one machine legitimately have several IP addresses?
Keep building