What does the "/" mean for an IP address? (firewall rule)


Feb 9, 2001
Hello All,

I've been wading through trying to find an answer but have been unsuccessful. I am trying to add some rules to my firewall (pfsense) and had a question about the "/" for an ipaddress for networks.

For the "network" option, the user is given the option to add /1-31 to the end of an IP address. For example,

What does the "/31" signify? I am trying to block a range of ip addresses and can't find what this means or if it will help me.

That was my initial thought but then why wouldn't it go to .255?

edit: posted at the same time as mikeblas

Thanks mikeblas!
So if I understand this correctly, would cover 4.2.xxx.xxx?
Dang, beet me to it. CIDR is great, now if only Cisco could make is it standard when putting IP addresses in instead of having to type the whole stupid thing out.
Cisco does a lot of things very oddly.