# 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 ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://ccie.nyquist.eu/ipv4/ipv4-addressing/ipv4-101.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.