You can also specify directly the IP or range of IPs for the sending mailservers for your domain using the ip4 mechanism. This can be done with either a specific IP, or the CIDR notation indicating a block of IPs. This is one of 2 mechanisms that cannot be used alone, without an argument following them. "ip4" on its own is not valid, SPF requires that you include the IP address in the data. Adding this to our example could result in something like the below.

There is also an "ip6" mechanism included in the SPF specifications for the new IP protocol, IPv6, which is in the process of being implemented. Unfortunately there is little detail on this since IPv6 is new and not yet well understood by much of the internet community. When it becomes common though, SPF is ready to support it.