Check out BOL section on Data Interface Pairs. If you have installed a client on say a production network, but then add a backup subnet, and don't want to go through configuring DNS for the new subnet, you can configure DIPs using Ip addresses between the client and media agent.
Here is the link: Data Interface Pairs
If you don't have multiple subnets but want to use IP address for the client, then you can always simply edit the client properties and change the client hostname to the IP address for that system.
Here is the link for changing client hostname in the properties: Name Management
But essentuially all that needs to be done is right click on the given client, select properties, and then click the "EDIT" button in the "Client Name" box. Replace the host name with the IP address desired.
That's about it...
Ernst F. Graeler
Senior Engineer III