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Beginner

Can a Layer2 network be comprised of more than one subnet?

If so, what are some good examples?   

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7 REPLIES 7
Cisco Employee

What are you trying to

What are you trying to achieve with this?

Could you please provide more info on what exactly you are lookig for?

Two different network/subnet in the same layer 2 switch without layer 3?

What kind of switch do you have?

Regards

Inayath

Beginner

If my ISP is providing

If my ISP is providing addresses to an entire city block, and they're all on the same /24 network, but the different buildings and businesses are chopped up using /29 networks.  Are we all not on the same L2 network?  I am really trying to dig deep and get a thorough understanding of L2.

For example 76.14.98.160/24

There are some 15 businesses all using some iteration of 76.14.98.160/24 e.g. 76.14.98.160/29 and 76.14.98.168/29 and 76.14.98.176/29.  

Is it when you can no longer see a device ARP'ing on your network that it's offically not on the same L2 network.  

VIP Expert

When frames are on a shared

When frames are on a shared (today, often it's only logically shared) "wire", or the same broadcast domain, they share L2.

You can have multiple networks on the same shared L2, depending on how hosts are configured, they will either ARP for an IP, that's not on their network, or send packets to a gateway IP, if destination isn't on the same network as the source.  I.e. if they ARP, there's no need to route, if they use gateway, then you'll need to route.  (The latter is the more common setup.)

Beginner

When you talk about Subnets,

When you talk about Subnets, You're probably talking about connectivity of them too. Computers in a subnet need to a layer 3 routing to communicate. Layer 2 connection uses Broadcasting and MAC Address , L3 needs Uni-casting and IP address and Default Gateway however.

Beginner

Hi,

Hi,

if I understand well your question L2 networks can be comprise of more that one subnet but whould need a route if they have to talk to each one.

Regards,

Beginner

So, what if I take a device

So, what if I take a device like bridge, from my ISP and I grab a single IP address from the public network, I am on the same L2 on edge of my ISP provided cable modem, but inside, we are not officially on a L3 network?  

VIP Expert

Yes.

Yes.

Possibly a common and useful example, when you need to migrate hosts on a VLAN subnet, often to provide a larger address block, or for some other need to re-IP.

In those cases, you have an interface with a primary IP ("new" subnet) and one secondary IP ("old" subnet).  By default, DHCP will work with primary IP, so for DHCP hosts, when they renew their leases, they will migrate to the "new" subnet.

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