The history is like this: In the beginning it used to be that a classful IP address block (such as 170.15.0.0 /16 or 197.100.2.0 /24) was assigned to a particular organization for its exclusive use. If that organization chose to subnet that block, and the organization's routers did not have a route listed for a particular subnet of that block, then a packet for that block would be dropped (even if a gateway of last resort had been configured). The logic being that if your organization doesn't know where that subnet is then nobody does, and thus the packet is dropped.
These days, ISPs have possession of various address blocks which they divvy up as needed to their customers. So its absolutely common for the various subnets of a particular classful address block to be split up amongst multiple organizations. So the ip classless command was used to get around this default behavior of the router. Recognizing the changed landscape of IP address allocation, ip classless became part of a router's default configuration starting with IOS version 11.3, permitting packets for any destination not explicitly listed in the router's routing table to utilize the router's configured gateway of last resort. Official information: http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_r /1rprt2/1ripadr.htm#1018036 Dan Garfield CCNP, CCDP, CCSI, MCSE, CNE, CTT, A+ 221 Oak Springs Drive San Anselmo, CA 94960 phone: (415) 453-5659 fax: (413) 691-6074 ""[EMAIL PROTECTED]"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > Brilliant! > > Pierre-Alex > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of > Bernard Omrani > Sent: Tuesday, December 25, 2001 7:21 PM > To: [EMAIL PROTECTED] > Subject: RE: IP classless command [7:30056] > > > For a simple explanation if IP classless, see: > > http://www.networkking.net/out/ipclassless.htm > > > Bernard > > > > -----Original Message----- > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] > > Sent: Tuesday, December 25, 2001 12:41 PM > > To: [EMAIL PROTECTED] > > Subject: Re: IP classless command [7:30056] > > > > IP CLASSLESS is a little hard to understand. Even Cisco is very vague > on > > this. Once a > > TAC engineer just told me that the command just makes routing better. > > Here > > is what I > > believe happens. > > > > If you don't have the "IP CLASSLESS" command defined in a Cisco > router > > then > > the router > > will not forward any packets towards a default route for any subnets > of a > > classfull > > network that the router thinks are local. For instance. > > > > Rtr A local net 10.1.0.0/24. > > 192.168.1.0 interconnects routers a and b > > RTR B (sends default network only to RTR A and RTR C) > > 192.168.2.0 interconnects routers b and C > > RTR C local net 10.2.0.0/24 > > Note how 10.0.0.0 is split by the 192.168.1.0 and 192.168.2.0 > networks. > > > > In the above example if RTR A gets a packet destined for 10.2.0.1, but > it > > does not have > > the "IP CLASSLESS" command defined then RTR A will drop the packets. > > > > If you include the IP CLASSLESS command then the packets will be > forwarded > > to the > > default router b. > > > > I hope this helps > > Mike Paulson > > Network Architect > > Infrastructure Design Systems LLP > > > > > > Hunt Lee wrote: > > > > > Can anyone please explain to me what is "ip classless" used for? I > > looked > > > it up on the Caslow book, and it states that by enabling IP > classless, > > it > > > allows one to override the contiguous subnet rule and allow the > router > > to > > > look for the longest match beyond the listed subnets. > > > > > > But I still don't understand what it means? Can anyone give me some > > > examples? > > > > > > Thanks in advance. > > > > > > Best Regards, > > > Hunt Lee > > > IP Solution Analyst > > > Cable & Wireless > > > > [GroupStudy.com removed an attachment of type text/x-vcard which had a > > name > > of michael.paulson.vcf] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=30122&t=30056 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
