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Community Member

Difficulty visualizing the following:

Hi,

This will make some of you laugh I am sure, but I just started learning and have the following questions:

PC1 (168.1.1.1) wants to send to another PC2 (165.1.1.1).

PC1 sends frame to router, router 1 decapsulates frame, makes new frame, sends to router 2 etc, until finally a router N can directly send to PC2.

Assume there are somewhere in the world other PC 3/4/5 with the same PC2 ip address (165.1.1.1), how does any one router know  which one of these is PC2. Somewhere I think MAC address has to be in the frame data otherwise but this is up to you guys to confirm or not.

Secondly, I understand routers send addres tables to neighbouring routers, and they again send that data to the next. However I have difficulty believing that my router ( at home for example) has all Ip addres data there is in the world. What do i not see?

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Cisco Employee

Difficulty visualizing the following:

Hi Jurgen,

Don't worry about us laughing - these are some very serious questions and there is absolutely no reason to laugh at anything.

Assume there are somewhere in the world other PC 3/4/5 with the same PC2 ip address (165.1.1.1),

If there is another PC2 somewhere in the world that shares the IP address 165.1.1.1 with another PC2 then the IP address 165.1.1.1 is not unique because it does not belong to a single station anymore. This would be an incorrect configuration because IP addresses are required to be globally unique.

how does any one router know  which one of these is PC2.

The fact is that it does not. Each router will simply route the packets to the closest PC that has the address of 165.1.1.1, even it that is not the true and intended recipient of the packet. Having conflicting IP addresses is simply something from which IP protocol can not recover. IP absolutely requires that IP addresses are unique, and if they are not, packets will be routed to the nearest owner of that address.

Somewhere I think MAC address has to be in the frame data otherwise but this is up to you guys to confirm or not.

No, the MAC addresses of the nearest recipient (and sender as well) is located only in the frame header and is used to direct the frame to the very next network card that is supposed to process the frame.

Secondly, I understand routers send addres tables to neighbouring  routers, and they again send that data to the next. However I have  difficulty believing that my router ( at home for example) has all Ip  addres data there is in the world. What do i not see?

Instead of knowing about each and every network out there, your home router simply has what we call a default route usually written down as 0.0.0.0/0. This default route serves the purpose of routing all packets which have not been dealt with by other existing entries in the routing table

You may already know that the routing table lookup is performed by taking the destination IP address, bitwise-ANDing it with the netmask in the routing table's row and comparing the result to the address of the network in that row. If a match is found, the packet will be routed using the information found in that entry. Now, notice that any IP address X.Y.Z.Q bitwise ANDed with 0.0.0.0 (the mask) would yield 0.0.0.0 (the network). Hence, the 0.0.0.0/0 network matches any destination, and is therefore a good entry capable of representing the entire internet.

Feel welcome to ask further!

Best regards,

Peter

5 REPLIES
Cisco Employee

Difficulty visualizing the following:

Hi Jurgen,

Don't worry about us laughing - these are some very serious questions and there is absolutely no reason to laugh at anything.

Assume there are somewhere in the world other PC 3/4/5 with the same PC2 ip address (165.1.1.1),

If there is another PC2 somewhere in the world that shares the IP address 165.1.1.1 with another PC2 then the IP address 165.1.1.1 is not unique because it does not belong to a single station anymore. This would be an incorrect configuration because IP addresses are required to be globally unique.

how does any one router know  which one of these is PC2.

The fact is that it does not. Each router will simply route the packets to the closest PC that has the address of 165.1.1.1, even it that is not the true and intended recipient of the packet. Having conflicting IP addresses is simply something from which IP protocol can not recover. IP absolutely requires that IP addresses are unique, and if they are not, packets will be routed to the nearest owner of that address.

Somewhere I think MAC address has to be in the frame data otherwise but this is up to you guys to confirm or not.

No, the MAC addresses of the nearest recipient (and sender as well) is located only in the frame header and is used to direct the frame to the very next network card that is supposed to process the frame.

Secondly, I understand routers send addres tables to neighbouring  routers, and they again send that data to the next. However I have  difficulty believing that my router ( at home for example) has all Ip  addres data there is in the world. What do i not see?

Instead of knowing about each and every network out there, your home router simply has what we call a default route usually written down as 0.0.0.0/0. This default route serves the purpose of routing all packets which have not been dealt with by other existing entries in the routing table

You may already know that the routing table lookup is performed by taking the destination IP address, bitwise-ANDing it with the netmask in the routing table's row and comparing the result to the address of the network in that row. If a match is found, the packet will be routed using the information found in that entry. Now, notice that any IP address X.Y.Z.Q bitwise ANDed with 0.0.0.0 (the mask) would yield 0.0.0.0 (the network). Hence, the 0.0.0.0/0 network matches any destination, and is therefore a good entry capable of representing the entire internet.

Feel welcome to ask further!

Best regards,

Peter

Community Member

Difficulty visualizing the following:

Hi Peter,

Thank you for these clear answers. I am only just beginning my CCENT study with no prior knowledge of the basics behind it all. So things will become clearer as I work my way through the chapters. I am looking forward to become part of this community since you made it clear that questions can be asked and support is there when you need it.

Best Regards,

Jurgen

Purple

Difficulty visualizing the following:

Hi Peter,

This is really a wonderful explanation, I'm definitely a fan of your way of explaining stuffs.

Regards

Alain

Don't forget to rate helpful posts.

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Super Bronze

Re: Difficulty visualizing the following:

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Posting

Jurgen, I just wanted to add to a few points to what Peter has noted.

First, IP uniqueness is per routing topology.  It's possible to "reuse" IPs, but there shouldn't be duplicates within the same topology.

The reason I mention this first point, you might have already seen the example of hosts using "private" IPs, such as 192.168.x.x, that are duplicated between different locations.  For example, if you examine many home (Internet) users, and check their IPs, they might have the same host IP address.  The trick, though, is this network address is translated (NAT'ed) to an Internet unique IP at the Internet boundary.  (This technique has been the principle reason why we haven't run out of IPv4 Internet addresses years ago.)

Second, routers usually do not have individual host IP addresses  Instead, they carry network addresses.  Additionally, especially on the Internet, routers often have aggregate addresses that covers ranges of networks.  Peter's mention of using the default route is the "largest" aggregate.

The trick to aggregate addresses is to assign consecutive networks so they can be aggregated.

For example, if router A has four networks attached to it, 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24, instead of telling router B about all four networks, router A might tell router B it has, 192.168.0.0/22.  Then router B only has to deal with looking at one entry rather than four when deciding whether a packet should be sent to router A.

Jurgen, the above is probably a bit more than you looking for in your original question, but as you learn more you'll fine there's even more to learn.

Community Member

Difficulty visualizing the following:

Hi Joseph,

IP uniqueness is per routing topology.  It's possible to "reuse" IPs, but there shouldn't be duplicates within the same topology.

This is what I thought but untill now wasn't covered.

Thank you for the extra info, i'll sleep better now.

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