ccna

Hi there,
I need help or advice on VLSM subnet assigment which is killing me.
I am cool with VLSM subnetting but i am confused with this question (a and b )
Any help
Thanks
You must devise a scheme allowing for more than 402 subnets (0 to 401) with 401 being the last subnet allocated for users. You are issued address 123.123.0.0/16. Note this is the correct public address, correctly allocated, not a misprint.
a) Using VLSM Provide a table listing:
i) Each additional sub-network address only for the subnets identified in figure 1
ii) The range of hosts for each subnet identified
iii) The broadcast address for each subnet identified

b.In addition, you must create an additional VLSM’ed subnet for the core (backbone) switched network. You must also create another set of suitable subnets for the serial point-to-point WAN links. For evidence, produce two tables that:

show all addresses for the core subnet
show all addresses for the point-to-point WAN connections

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olongos

show all addresses for the core subnet
show all addresses for the point-to-point WAN connections

benbullock1992

Hello,

I believe you and I are attempting the same assignment.

I'm also struggling on this part and cannot proceed until I have solved it.

Do you have any tips/pointers?

I think where I'm stuck most is the connections between the routers and the LAN switches. Nowhere in the question does it mention creating subnets for these.

I'd appreciate any help.

Thanks,

Ben

Neil86

You need 405 total subnets to cover the 402 user subnets and the 1 core and 2 WAN subnets. The smallest amount of bits you need to borrow to get a minimum of 405 subnets is 9 bits (9²=512 subnets).

Your original network 123.123.0.0 /16 (nnnnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh) 1 network with 65,534 host addresses.

It will then be 123.123.0.0 /25 (nnnnnnnn.nnnnnnnn.nnnnnnnn.nhhhhhhh) 512 (2^n(borrowed)) subnets with 126 host addresses each (2^h-2).

So, for the user subnets 0-401, your subnets will be in increments of 128:

123.123.0.0 - 123.123.0.127
123.123.0.128 - 123.123.0.255
123.123.1.0 - 123.123.1.127
123.123.1.128 - 123.123.1.255
.
.
.
123.123.255.128 - 123.123.255.255 (the last subnet #512)

The first 402 subnets are allocated for the user subnets. You need to identify subnets 0-8, 399, 400, and 401.

Next, you have to make a subnet for the core (8 host addresses needed) and a subnet for each serial WAN link (2 hosts each).

You have satisfied the user subnets requirements with the first 402 subnets, so after that you can change the mask for the other requirements.

Your last host subnet (#402), if my math is correct, is 123.123.200.128 - 123.123.200.255. After that, you can change the mask.

For the core, starting at 123.123.201.0, you need 8 host addresses. Change the mask to a /28, leaving you 4 host bits (2⁴-2 = 14 usable addresses).

Core subnet = 123.123.201.0 - 123.123.201.15 /28

WAN Links - starting after the core network above, you need 2 more subnets with 2 hosts address each for the serial links. Change the mask to a /30, leaving you 2 host bits (2²-2 = 2 host addresses).

WAN link 1 subnet = 123.123.201.16 - 123.123.201.19 /30
WAN link 2 subnet = 123.123.201.20 - 123.123.201.23 /30

I'm no guru at this, so the above could be off. But I enjoy attempting for the practice.

Subnetting can be tricky, especially in 2nd and 3rd octet, so get familiar with the math to figure out the number of subnets and hosts. Also, get familiar with the notational value of the bits in each octet.

Also, no one is going to know ALL 512 user subnets from above off the top of their head. Use subnet calculator tools to help with the process.

benbullock1992

Hi Neil,

Thanks for your reply, this helps a lot!

Can you tell me, for the core subnet, why you need 8 host addresses? Is it 2 addresses per router (1 for the link back to the Somerset switch and 1 for the link to the LAN switch)?

If so, I’ve tried this in Packet Tracer and Cisco IOS is telling me I cannot do this as I’m not allowed to have addresses from the same subnet on different interfaces on the router. Basically each router interface needs a different IP address from different subnets.

If this is true, does the core subnet not just need 4 host addresses (1 per router for the interface linking back to Somerset) and a further 4 subnets for the router to LAN switches?

I hope this makes sense and you can help.

Thanks,

Ben

Neil86

Hi Ben,

After looking at the diagram again, I overlooked the fact that Somerset is a switch. The four router interfaces going back to Somerset in area 0 would all be in the same subnet, according to the instructions and if I'm understanding correctly, but instead of it being a /28 giving you 14 host addresses, it would be a /29 giving you 6 host addresses:

Core subnet: 123.123.201.0 - 123.123.201.7

This would also change the 2 serial WAN link subnets, same mask.

serial WAN 1: 123.123.201.8 - 123.123.201.11
serial WAN 2: 123.123.201.12 - 123.123.201 15

The router interfaces facing the user subnets, again if I am remembering correctly, would need subinterface commands to allow inter-vlan communication because it is a ROAS design.

benbullock1992

Hi Neil,

Thanks for this! This makes perfect sense now.

Please could you elaborate on what I need to do for the ROAS design or point me in the direction of a good tutorial?

Does ROAS require further subnets (IP addresses on the Router interfaces?

Thanks,

Ben

Neil86

Hi Ben,

Glad you found it helpful. I enjoy the practice.

Subinterfaces are logical divisions of a physical interface. So, in your assignment, each router interface would have 3 subinterfaces. Each subinterface would be assigned an address within the subnet that is assigned to it, and assigned to the vlan that the corresponding subnet is part of.

Here is some good material on the concept. It actually resembles your assignment somewhat:

https://learningnetwork.cisco.com/docs/DOC-23481