Expanding subnet

[ronstengel]

I have a 220 user network (192.168.1.1/24) and will soon be adding many new users > 300. How can I add the additional users without changing the 192.168.1.x/24 network?

Thanks in advance,

[Edison Ortiz]

You can introduce a layer 3 device (a dedicated router or a layer 3 switch) and create a separate subnet, let's say 192.168.2.0/24

[routeflap]

You can add a secondary IP address for an additional network like 192.168.2.0/24. See below:

Router#sho run int fa1/0

Building configuration...

Current configuration : 164 bytes

!

interface FastEthernet1/0

ip address 192.168.2.1 255.255.255.0 secondary

ip address 192.168.1.1 255.255.255.0

end

Router#

If the users are receiving their IP addresses dynamically (i.e. DHCP) you could switch over to a /23 network like 10.1.0.0/23. This would give you 10.1.0.2 - 10.1.1.254 for the user?s IP addressing assuming 10.1.0.1 would be the router's IP address.

Also note that depending on your user?s traffic and your layer 2 network design you could start to run into problems once you get over 200+ users.

Brian Dennis, CCIE #2210 (R&S/ISP-Dial/Security)

bdennis@internetworkexpert.com

Internetwork Expert, Inc.

http://www.InternetworkExpert.com

Toll Free: 877-224-8987

Direct: 775-745-6404 (Outside the US and Canada)

[weerapatr]

Other question when you add a secondary ip on your network then subnetA can talk to subnetB and vise versa?

Let's say

GWA = 192.168.1.1 --> Primary IP address

GWA = 192.168.2.1 --> Secondary IP address

PCofNetA <---> GWA <---> GWB <---> PCofNetB

TIA

[Richard Burts]

Weerapatr

Yes the two subnets can talk to each other. There might be some variability in how they do this. (1) Since both subnets are in the same broadcast domain they will hear each other's ARP requests and ARP responses and might learn each others MAC addresses and can communicate directly. (2) If a device in subnetA wants to communicate with a device in subnetB it might send the packets to the router and the router would forward the packet and the router would also generate an ICMP redirect packet to the originating device indicating that the device can communicate without needing the router to forward. If the device honors the redirect then the devices will communicate directly. (3) the devices might send their traffic to their default gateway (the router) since they believe that they are in different subnets and the router would forward traffic between the devices. This is the least efficient method, but it does work and communication would be successful.

One thing to note in your posting is that GWA and GWB are the same device. Your posting draws them as if they were separate, but they are the same device. So it is really:

PCofNetA <-->GWA/B <--> PCofNetB

HTH

Rick

[weerapatr]

Many thank for clarification Rick

[weerapatr]

Hi rick I need added some information about icmp redirect.

When Are ICMP Redirects Sent?

Cisco routers send ICMP redirects when all of these conditions are met:

1. The interface on which the packet comes into the router is the same interface on which the packet gets routed out.

2. The subnet or network of the source IP address is on the same subnet or network of the next-hop IP address of the routed packet.

3. The datagram is not source-routed .

4. The kernel is configured to send redirects. (By default, Cisco routers send ICMP redirects. The interface subcommand no ip redirects can be used to disable ICMP

redirects.)

For example, if a router has two IP addresses on one of its interfaces:

interface ethernet 0

ip address 171.68.179.1 255.255.255.0

ip address 171.68.254.1 255.255.255.0 secondary

If the router receives a packet that is sourced from a host in the subnet 171.68.179.0 and destined to a host in the subnet

171.68.254.0, the router does not send an ICMP redirect because only the first condition is met, not the second.

The original packet for which the router sends a redirect still gets routed to the correct destination.

ref. http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094702.shtml