SPF/term/cidr-length
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<cidr-length> refers to the CIDR (Classless Inter-Domain Routing) convention for specifying blocks of IP addresses.
SPF uses three possible versions of this:
- <ip4-cidr-length>: defaults to "/32"
- <ip6-cidr-length>: defaults to "/128"
- <dual-cidr-length>: includes at least one of <ip4-cidr-length> and <ip6-cidr-length>, and possibly both
- If <ip6-cidr-length> appears, it must be prefixed with "/".
- If both appear, <ip4-cidr-length> must be first.
The official and almost-comprehensible RFC documentation has this to say (Section 5.6):
ip4 = "ip4" ":" ip4-network [ ip4-cidr-length ] ip6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
ip4-cidr-length = "/" ("0" / %x31-39 0*1DIGIT) ; value range 0-32
ip6-cidr-length = "/" ("0" / %x31-39 0*2DIGIT) ; value range 0-128
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = qnum "." qnum "." qnum "." qnum
qnum = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
; as per conventional dotted-quad notation, e.g., 192.0.2.0
ip6-network = <as per Section 2.2 of [RFC4291]> ; e.g., 2001:db8::cd30
The <ip> is compared to the given network. If CIDR prefix length high-order bits match, the mechanism matches.
If ip4-cidr-length is omitted, it is taken to be "/32". If ip6-cidr-length is omitted, it is taken to be "/128". It is not permitted to omit parts of the IP address instead of using CIDR notations. That is, use 192.0.2.0/24 instead of 192.0.2.