2 Vlans with 25 devices each with same network

[chandra_rc16]

Hi,

Can someone please help me for the below scenario ?

Assume that i have 50 PCs in my organization and only two groups of people working called as Voice & Non-voice. And the IP addressing i want to use is

10.0.0.0/26.

For voice group i have assigned IP address range from : 10.0.0.1/26 to 10.0.0.25./26

For non-voice group i have assigned IP address range from: 10.0.0.26/26 to 10.0.0.50/26

I have placed all the voice PC's in Vlan10 and nonvoice PCS's in Vlan20.

And i have assinged 10.0.0.62/26 for my router.

And i have assigned 10.0.0.55/26 for the Vlan10

When i'm trying to assign 10.0.0.56/26 for the Vlan20.. it is alerting me that it overlaps.

Can someone please let me know where i'm wrong ?

Regards,

Chandu

[vmiller · Accepted solution]

You are splitting the /26 into 2 /27's. you need to adjust your mask accordingly

subnet               mask                   inverse         size          host range                    broadcast

10.0.0.0         255.255.255.224    0.0.0.31         30    10.0.0.1  to  10.0.0.30      10.0.0.31
10.0.0.32       255.255.255.224    0.0.0.31         30    10.0.0.33  to  10.0.0.62    10.0.0.63

[chandra_rc16]

Thank you.

But i have a doubt. So when we are planning IP Addressing. Do we need to consider every VLAN as a host ? Because we need to assign an IP Address to it.

And why the VLAN's need Layer 3 IP address. Because while we configure "router on a stick" we give the command "encapsulation dot1Q ". Can't the routing can be done only with the Vlan ID ?

And even we have to assign an IP address to the sub-interface. So it seems like in planning IP Addressing we need to think of both the vlan ip and the ip for the routers subinterface. Aren't we wasting two IP addresses ?

And which address will network admins usually assign to the VLAN's ? Will they pick in randomly or first address or last address like that ?

Thanks in Advance.

I'm new to networking, please don't mind if my question seems to be silly.

Regards,

Chandu

[Simen Ringstad]

Routers use layer 3 addressing to make routing decisions. VLAN ID is a layer 2 tag. When the ethernet frame arrives at your router interface with a VLAN tag the tag is stripped away before it is sent to the actual routing engine.

You don't have to have a subnet for each VLAN but it is best practice and in 99 out of 100 times it is the way to go. Yes you will use one more IP-address, but are you really short on RFC 1918-addresses?

[vmiller]

To follow up on Simens reply.

If Possible, consider each VLAN a subnet. it removes confusion (for you) in that sense, each VLAN needs a route to the rest of the world... an IP address. which should be in the same subnet as the VLAN.

Take my first example above, I would make my router interface the 10.0.0.1 address, It will handle routing for the rest of the subnet.

[chandra_rc16]

Thanks Simen and vmiller for clearing my confustion.

vmiller i'll implement your example and ping you for any difficulties.

Thank you both.

[vmiller]

No Problem.

1. Keep things simple; 1 vlan = 1 subnet.

     within the subnet/vlan use the first host for what ever routed interface you use.

     save a couple of addresses for printers, use the same range for each user subnet

          first host - router 9or sub interface

          hosts 2 thru 5 - printers

          5 and above  anything else in the vlan.

just get one to work for now....

[chandra_rc16]

Thank a lot. It worked for me.

And is it neccessay for us to remeber the RFC numbers to pass CCNA R&S exam ?

Regards,

Chandu

[vmiller]

Focus on fundamentals. quoting RFC's chapter line and verse is pretty meaningless.

1. Subnetting you need to know this backwards and forwards. be capable of working subnetting problems

     with pencil & paper.

2. understand the mechanics of routing, arp lookup, and basic switch operation.

3. understand the OSI model and how it relates to internetworking issues.

There is a blueprint on the ciso site.

[chandra_rc16]

Thanks a lot.