Firewall Wizards mailing list archives
RE: cisco config help
From: jan () nil si
Date: Mon, 28 May 2001 12:57:12 +0200
"Behm, Jeffrey L." <BehmJL () bvsg com> To: firewall-wizards () nfr com Sent by: cc: firewall-wizards-adm Subject: RE: [fw-wiz] cisco config help in () nfr com 25.05.2001 22:40
My understanding of this issue is that the mask in the access list is not really a subnet mask, as most people think of them. It is more just a mask that tells how many addresses to include in the range
for example, the next three statements refer to the private IP ranges 10.0.0.0->10.255.255.255, 172.16.0.0->172.31.255.255, and 192.168.0.0->192.168.255.255:
access-list 104 deny ip any 10.0.0.0 0.255.255.255 access-list 104 deny ip any 172.16.0.0 0.15.255.255 access-list 104 deny ip any 192.168.0.0 0.0.255.255
Hi, be careful - this seems nice, but is definitely NOT the way to write ACLs. The nice additive properties which you see in the above example are an EXCEPTION, which only happens if you break a contiguous prefix on natural borders (i.e. keep dividing it by two) and never cross that border with a single matching rule (i.e. ACL line). One should never, ever, think about ACLs as ranges, except if you are quite at home with binary arithmetics and know when this shortcut can be applied. As Ryan pointed out, they are the binary inverse of a subnet mask and this is the safest way to treat them :) Try to match 172.16.0.0-172.63.255.255 with a SINGLE rule and you will see. And then there are non-contiguous masks... :) CAVEAT: PIX Firewall access lists use netmasks instead of wildcards. I guess Cisco thought this will be less prone to misconfigurations, when their PIX customers transitioned from the old syntax (conduit/outbound, which used netmasks) to the new ACLs. Cheers, Jan Jan Bervar Specialist za podatkovne komunikacije, CCIE #2527 Consulting Engineer NIL Data Communications, Einspielerjeva 6, 1000 Ljubljana, Slovenia Phone +386 1 4746 500 Fax +386 1 4746 501 http://www.NIL.si _______________________________________________ firewall-wizards mailing list firewall-wizards () nfr com http://www.nfr.com/mailman/listinfo/firewall-wizards
Current thread:
- cisco config help Stuart Clark (May 25)
- Re: cisco config help Luca Berra (May 26)
- Re: cisco config help Ryan Russell (May 26)
- Re: cisco config help Mikhail Evstiounin (May 26)
- <Possible follow-ups>
- RE: cisco config help Behm, Jeffrey L. (May 26)
- RE: cisco config help jan (May 30)