Each of these checks requires an exact match, but this may not always be desirable. One may need to specify a range of IPs from which a legitimate mailserver may be sending, for example, so SPF allows one to specify a "cidr-length" at the end.

CIDR, or Classless Internet Domain Routing, is a notation used for specifying a range of IP addresses. This involves the IP address followed by a slash and a number indicating the size of the block. The number used indicates the proportion of the last section of the address used for the block. The larger the number the smaller the block, so a block like 23.17.42.93/24 represents a block of 256 host addresses which would all begin with 23.17.42, while a /27 includes 32 addresses, and a /32 is one specific host address.

In SPF this allows one to specify a block based on the A, MX, or PTR records, so that any server within that group can send email for the domain. If you enter something like a:example.com/27 that would allow any IP address within the 32 address block based on the IP for example.com.