Classless vs Classful

altjx

Good morning all,

I have been studying for the ICND1 for the past month and I must admit, I am very familiar with at least 95% of the material covered. I have taken the Cisco course in College about 2 years ago, but since then, I've been going the Microsoft route to increase my performance at my last job. Recently, I've started working in IT Security, where I've always wanted to be. CCNA has been the most recommended certification to break more into this field.

I have done countless (100s) of Cisco Packet Tracers, Boson NetSim labs, hands on labs (in school for 2 years), and I'm an expert with subnetting and IOS CLI. The one thing that I've pretended to avoid for awhile was the terms "classless" and "classful". I know RIP is a Classful protocol and RIPv2 is classless. I've done over 500 practice questions and feel over prepared for the exam. The exam is scheduled for next Monday.

Although I know that, I still don't completely have a clue. I've learned (from practice test) that if using RIP, CIDR notation and multicast updates won't be shown in the debug ip rip outputs; RIPv2 updates will.

Can someone please help explain a little more detail between the two terms in a more simpler term? The book talks about it, and I've googled here and there, but I still don't completely understand the difference other than the two.

Thanks.

alan2308

RIP is a classful protocol because when it sends its updates, it sends the network but not the subnet mask for each route. It was designed to work with classes, not subnets. RIPv2, is a classless protocol because when it sends its updates, it sends both the network and the subnet mask for each route. If the mask is not sent, then assumptions have to be made, so you cannot use different masks for subnets in the same classful boundary. For example, if you have multiple subnets within the 10.0.0.0/8 range (a class A network), you cannot use a /24 for one subnet, a /25 for another, and /26 for yet another with RIP because the subnet masks are not included in the updates, so routers that are not directly connected would have no idea what all of the masks are.

Now with RIP, if you use the same mask everywhere, then you'll be ok. If the router has a network within the 10.0.0.0 range directly connected with a /24 mask, then it will assume everything within the 10.0.0.0 range will also be using a /24.

altjx

alan2308 wrote: »

RIP is a classful protocol because when it sends its updates, it sends the network but not the subnet mask for each route. It was designed to work with classes, not subnets. RIPv2, is a classless protocol because when it sends its updates, it sends both the network and the subnet mask for each route. If the mask is not sent, then assumptions have to be made, so you cannot use different masks for subnets in the same classful boundary. For example, if you have multiple subnets within the 10.0.0.0/8 range (a class A network), you cannot use a /24 for one subnet, a /25 for another, and /26 for yet another with RIP because the subnet masks are not included in the updates, so routers that are not directly connected would have no idea what all of the masks are.

Now with RIP, if you use the same mask everywhere, then you'll be ok. If the router has a network within the 10.0.0.0 range directly connected with a /24 mask, then it will assume everything within the 10.0.0.0 range will also be using a /24.

Now that makes sense . Thank you so much Alan for that information.

cisco_kidd

altjx wrote: »

Good morning all,

I have been studying for the ICND1 for the past month and I must admit, I am very familiar with at least 95% of the material covered. I have taken the Cisco course in College about 2 years ago, but since then, I've been going the Microsoft route to increase my performance at my last job. Recently, I've started working in IT Security, where I've always wanted to be. CCNA has been the most recommended certification to break more into this field.

I have done countless (100s) of Cisco Packet Tracers, Boson NetSim labs, hands on labs (in school for 2 years), and I'm an expert with subnetting and IOS CLI. The one thing that I've pretended to avoid for awhile was the terms "classless" and "classful". I know RIP is a Classful protocol and RIPv2 is classless. I've done over 500 practice questions and feel over prepared for the exam. The exam is scheduled for next Monday.

Although I know that, I still don't completely have a clue. I've learned (from practice test) that if using RIP, CIDR notation and multicast updates won't be shown in the debug ip rip outputs; RIPv2 updates will.

Can someone please help explain a little more detail between the two terms in a more simpler term? The book talks about it, and I've googled here and there, but I still don't completely understand the difference other than the two.

Thanks.

Excellent Explanation!

altjx

I have another question regarding classless vs classful.

What's the difference between the two when it comes to using the default route? According to the book:

Classful Routing - The default route is only used if the classful network in which the packet's destination address resides is missing from the routing table.
Classless Routing - The default route is always used for packets whose destination IP address does not match any other routes.

What's the difference? To me, it sounds like both use the default route if there's no match in the routing table. Not sure what I'm missing.