"Hylton Conacher(ZR1HPC)" <conacher.co.za!hylton> said (on 2006/08/21):
...
What further confuses me is that I read on Google that vlans can also
have subnets.
Could someone define each for me and the list and also why one is more
secure than the other.
Tnx
Hylton
You're familiar with the 7-layer OSI model? Vlans are a layer 2 entity;
Subnets are layer 3. A vlan defines the broadcast domain of an ethernet
frame (as one commonly-used layer 2 example). A subnet defines the
broadcast domain of an IP (for example) packet. Collision domains are
the synonymous layer 1 concept.
Vlans and subnets are apples and oranges, so a security comparison between
the two is meaningless.
To illustrate, suppose A and B are on the same subnet, but different
vlans. When A wants to talk to B, it will send an arp request with
a broadcast mac address. All computers on A's vlan will see this arp
request. Because B is on a different vlan, it will not see the arp request.
Instead, we need a router (R, let's say) who has interface(s) on A's vlan
and B's vlan. R will have to answer A's arp with its own mac address. A
will create a frame with R's mac as destination, containing an IP packet
with B's IP address as destination. The frame will reach R, who will
change the destination mac address to B's mac and send it out on B's
vlan. The process works in reverse when B wishes to talk to A.
For the sake of sanity, many organizations align their layer 2 vlans
with their layer 3 subnets. This makes it easier to conceptually grasp
the network's layout.
There's a lot more that could be said about network design, but I'll end it
here having reasonably answered your question. Trying to combine layer 2
and layer 3 has to be one of the most common confusions afflicting young
(well, new to the field, at least) network minions.
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