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Home > Archive > CCNA > April 2003 > VLSM question
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| soccer4net 2003-04-24, 12:02 pm |
| Hoping someone can explain this, or point me to a good tutorial on it.
I understand how vlsm can be used to break an address into differerent sized networks.
I've read the3com article on this which is very good. But theres one thing I don't get, where do the router/serial segments fit into this.
If you have your network numbers large at the top and going in descending order to fit each network as snugly as possible, it seems like either segments connecting high level routers either have way to many addresses or none at all(you only need two).
Lets say youv'e got 3 networks connected with 3 routers. Capital letters=Router lower case = subnet. And the address: 192.168.1.0/24
A
B C
a b c
Subnet a=100h b,c=50h each.
Say were supporting subnet 0 so we subnet to /27 .0 for RB and .128 for RC
then subnet .128 to /26, leaving .128/26 for subnet b and .192/26 for subnet c.
This is how I understand most VLSM explanations, but this leaves no allotment for addresses between routers A, B and C. Plus I just made a simple example things are further complicated when you have multiple layers of two host segments between routers.
Such as:
A
B C
D E F G
a b H I h i j k
cd ef
Maybe I've just got a mental blockage somewhere in there, but if someone could enlighten me I'd appreciate it. | |
| darronb 2003-04-24, 1:42 pm |
| Does this type of question come up in the CCAN exam | |
| anchor40 2003-04-24, 2:55 pm |
| You are absolutely correct. With those quantities of required hosts, the entire range of addresses would be utilized.
For the exam, though, you might see the question along these lines, just with smaller quantites of hosts per segment OR more addresses that can be allocated (192.168.2.0/23) to satisfy the hosts.
They also like to ask questions opposite from what you're used to seeing, just to make sure you understand the question. For example, if the customer asks for 4 segments of 25 hosts per segment, how many additional segments could be assigned in the future?
2: a /27 = six segments of 30 hosts.
Also, unless specified, ip subnet-zero is not assumed.
HTH... | |
| djmaplethorpe 2003-04-24, 5:13 pm |
| quote:
Originally posted by anchor40
For the exam, though, you might see the question along these lines, just with smaller quantites of hosts per segment OR more addresses that can be allocated (192.168.2.0/23) to satisfy the hosts.
HTH...
I don't mean to question but after what I've read and I quote "When using subnet masks to break up a Class C network, there are only five valid netmasks:
Netmask bits networks host
255.255.255.192 26 2 62
255.255.255.224 27 6 30
255.255.255.240 28 14 14
255.255.255.248 29 30 6
255.255.255.252 30 62 2"
How can you use a mask smaller than the classful address? If I am wrong please correct me. | |
| edmonds_robert 2003-04-25, 9:00 am |
| What a lot of people will do is set aside an entire subnet for use on serial lines, then use a /30 mask (255.255.255.252) to break that subnet down further. For example, on my network we use 172.16.0.0/24 networks. I have set aside 172.16.254.0/24 for serial lines. So, my first one is 172.16.254.1 and 172.16.254.2, my second is 172.16.254.5 and 172.16.254.6, and so on. | |
| soccer4net 2003-04-25, 10:01 am |
| quote:
Originally posted by djmaplethorpe
I don't mean to question but after what I've read and I quote "When using subnet masks to break up a Class C network, there are only five valid netmasks:
...
How can you use a mask smaller than the classful address? If I am wrong please correct me.
For the CCNA that's all you need to know. This is going beyond the scope of this exam, but with CIDR/VLSM and subnet 0, yes you can subnet by 1 bit, if you want the technical details lookup the rfc articles on the subject. Modern equipment does support this, to varying extents. Anyway for the exam you probably won't need to worry about any of this. | |
| soccer4net 2003-04-25, 10:09 am |
| Thanks for clearing that up guys. I figured they were leaving something out. I guess they expect you to assume that, but its really an important point, reserving the right size subnet to split between router to router segments.
The descending subnet diagrams only apply to host segments connected routers, not the router to router segments. When you have a limited address space, this adds an interesting twist to the puzzle, I'll have to look at this closer.
Thanks again for your input. |
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