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Curiosity

I was browsing and somehow landed to a domain’s spf record, I was surprised to see Hostname with CIDR and thought that the SPF record is wrong because we have always seen CIDR with IP addresses not with the hostnames.

v=spf1 a:rpc.boldchat.com/23 include:_spf.learntechfuture.com  ~all




RFC Reference

I started the research and first reference was to look into SPF RFC, Here is the Screenshot from https://tools.ietf.org/html/rfc7208

It means that for Similarly for rpc.boldchat.com/23. In this case, Sender IP is matched against the records of rpc.boldchat.com and if multiple IPs exists then each one will be compare.

Finally, It proves that hostname with CIDR can also be added to the SPF Record.