Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

2 Vlans with 25 devices each with same network

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

Regards, Chandu
1 ACCEPTED SOLUTION

Accepted Solutions
Gold

2 Vlans with 25 devices each with same network

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

9 REPLIES
Gold

2 Vlans with 25 devices each with same network

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

New Member

2 Vlans with 25 devices each with same network

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

Regards, Chandu
New Member

2 Vlans with 25 devices each with same network

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?

Gold

2 Vlans with 25 devices each with same network

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.

New Member

2 Vlans with 25 devices each with same network

Thanks Simen and vmiller for clearing my confustion.

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

Thank you both.

Regards, Chandu
Gold

2 Vlans with 25 devices each with same network

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....

New Member

2 Vlans with 25 devices each with same network

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

Regards, Chandu
Gold

2 Vlans with 25 devices each with same network

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.

New Member

2 Vlans with 25 devices each with same network

Thanks a lot.

Regards, Chandu
164
Views
0
Helpful
9
Replies