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New Member

Expanding subnet

I have a 220 user network ( 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,

  • WAN Routing and Switching
Hall of Fame Super Bronze

Re: Expanding subnet

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


Re: Expanding subnet

New Member

Re: Expanding subnet

You can add a secondary IP address for an additional network like See below:

Router#sho run int fa1/0

Building configuration...

Current configuration : 164 bytes


interface FastEthernet1/0

ip address secondary

ip address



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

Internetwork Expert, Inc.

Toll Free: 877-224-8987

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

New Member

Re: Expanding subnet

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

Let's say

GWA = --> Primary IP address

GWA = --> Secondary IP address

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


Hall of Fame Super Silver

Re: Expanding subnet


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



New Member

Re: Expanding subnet

Many thank for clarification Rick

New Member

Re: Expanding subnet

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


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

interface ethernet 0

ip address

ip address secondary

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