# Given the IP address 22.181.0.0/21, create subnets starting from this address

Given the IP address, 22.181.0.0/21, subnet the network for the following offices, based on the number of devices for them (The first two columns are given. I've calculated third, fourth and fifth column myself):

``````Off. Dev.   Total Bits Slash
A    47     49    6    /26
B    255    257   9    /23
C    123    125   7    /25
D    71     73    7    /25
E    63     65    7    /25
F    36     38    6    /26
G    24     26    5    /27
``````

If the problem was just this, I'd solve it this way: However, there is one extra condition in the task:

All offices B-G should be connected only to A

This task puzzles me because so far, I haven't solved any problems with such requirements. Moreover, the network, which is supposed to be in the root, is much smaller than those at the leafs. How do I modify my solution, so that it fits the criteria?

• Could I get some feedback on how to improve my question? Mar 26, 2022 at 17:52
• Here's a suggestion. Ask your teacher for help if you don't understand your homework assignment. Mar 26, 2022 at 18:07
• @LPChip Unfortunately, I've already done that. They provided information on how to solve such tasks but without the additional requirement, which I cited. I've also researched the problem in the provided book to no avail. In fact, those two actions were easier to do than translate the task from my native language into English, type it, create a diagram from my notes and then finally post everything here. I always search the internet before asking a question. Mar 26, 2022 at 18:36
• so. to me, the requirement "All offices B-G should be connected only to A" implies a star topology, where each sub-network B-G exist connected directly to A. en.wikipedia.org/wiki/Star_network . it seems like you are assuming that internetwork connections are more complicated than they are. most of the right side of your diagram is unnecessary. you don't need routers for each smaller subnet mask (/23, /24, /25, /26, etc). you can directly connect 22.181.4.0/27 to the router controlling 22.181.0.0/21. additionally you have A as a leaf node instead of the root of the tree. Mar 27, 2022 at 9:07
• @FrankThomas Thanks for your contribution. The right side of the diagram is indeed unnecessary. What I meant with this diagram is how I split the IP networks into two parts recursively until I reach a small enough domain to assign an office to it. Sorry for the ambiguity. As for A in the leaf, the accepted answer explains that the networks are, in fact, peers. Thus, A is equal to B-G. Therefore,the condition `All offices B-G should be connected only to A` means we just have to take into account some extra space for the connections from A to B-G. That's what I've understood from the answer. Mar 27, 2022 at 13:27

Moreover, the network, which is supposed to be in the root, is much smaller than those at the leafs.

That doesn't matter. It only needs to be large enough to fit the gateways. There's no reason for it to be larger: it doesn't contain all of the leaf networks, it only connects them.

In other words, there's no "inner/outer" relationship between subnets, they connect as peers, and hosts in B-G do not automatically become members of A – only the subnet gateways are part of both networks they connect.

Since you're connecting 6 networks using A as the "root", even if each of them has its separate gateway¹, that's still only 6 addresses (maybe 7 if there's also an Internet gateway) that must fit in A next to its own hosts. Since your table has 47 devices in A, a /26 will do.

The Internet will have a route that covers all networks at once (e.g. a /21 route), but the size of a route doesn't need to correspond to the size of any particular network. So the /21 route could just as well be directed to gateway WAN–A even though A itself is only a /26; the gateway will just have additional routes for the other subnets through gateways A–B, A–C, and so on.

¹ (In practice there could be even fewer, for example, if B/C/D were just VLANs within the same building, then it could be a single gateway connecting all three subnets to A...)

• Thanks for you explanation. I think I'm starting to understand now. The visual use of trees does not represent a hierarchy, rather only the algorithm on how to partition the network into smaller pieces. Therefore we could think of the resulting subnetworks as neighbouring spaces in an array, as equals. Thus, the only thing to change in my solutuon is add 6 addresses that represent the connections A-B, A-C,... A-G in A. Thus, 47+6 < 64 and, as you said, /26 still works. Have I understood your explanation correctly? Mar 27, 2022 at 13:13
• Yes, the tree is purely for partitioning the address space. Although the hierarchy remains relevant if you want to aggregate routes (e.g. if C & D happen to be in the same building and sharing a gateway, routes towards their /25's could be represented as a single /24 route, whereas D & E couldn't be). Also, if you draw the network as a graph, the "connections" represented by these 6 addresses (i.e. the gateways) would still be nodes participating in both networks, not edges connecting them. Mar 27, 2022 at 13:34
• Aha. And in this case, the gateways should be /30 nodes, right? Mar 27, 2022 at 13:39
• No, in this example they're still /26 on the network A side and /23 on the network B side, etc. (All nodes in a network use the same prefix length.) When /30 or /31 is used for connecting networks, it would be a separate address range, acting as a network in itself (aka "linknet" or "transfer network"), so instead of network B's gateway being also member of A, you'd have `network B <-> gateway <-> a /30 link network <-> gateway <-> network A`. It's possible that your assignment expected you to allocate six /30's for that purpose, too. Mar 27, 2022 at 14:01
• Thank you so much for your patience! You really helped me a lot. I appreciate your effort Mar 27, 2022 at 14:20