> For the complete documentation index, see [llms.txt](https://ccie.nyquist.eu/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ccie.nyquist.eu/ipv4/ipv4-addressing/ipv4-101.md).

# IPv4 101

## Setting an IP Address

```
R(config)#ip address IP-ADDR NETMASK
```

Any combination of IP-ADDR and NETMASK can be used as long as the **host** portion of the address is not all zeros. One exception is allowed, when using a /31 mask.\
By default, the router will accept combinations that result in a **subnet** portion of the address with all zeros, but this behavior can be disabled using:

```
R(config)#no ip subnet-zero
```

### Secondary IP Addresses

Multiple IP Addresses can be configured on an interface using the **secondary** keyword:

```
R(config-if)# ip address IP-ADDR NETMASK secondary
```

### IP Unnumbered

This feature allows a router to use the IP address of another interface instead of using a new IP Address. It can only be configured on point-to-point interfaces

```
R(config-if)# ip unnumbered INTERFACE
```