vlans and subnets

5 pts.
Tags:
Subnet
VLAN
would different vlans have different subnets?

Software/Hardware used:
any

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yes. A subnet: part of the IP address space, eg 192.168.1.0/255.255.255.0, 10.1.1.1/255.255.255.0 (this one especially, 10.x.x.x networks normally have 255.0.0.0 as the subnet).

A VLAN: A “Virtual” LAN – a section of ports on a/many switch/es that are act as if they are their own seperate LAN – can have many different IP subnets on it as VLANs are not based on IP’s.

A subnet is a layer 3 term. Layer 3 is the IP layer where IP addresses live.

A VLAN is a layer 2 term, usually referring to a broadcast domain. Layer 2 is where MAC addresses live.

Consider:

On a cheap normal switch, there is just one single broadcast domain – the LAN – containing all the physical ports.

On a more expensive switch, you can configure each phycical port to belong to one or more virtual LANs (VLANs). Each VLAN has its own broadcast space and only other ports on the switch assigned to the same VLAN as you get to see your broadcasts.

Most commonly, broadcast traffic is used for ARP so that hosts can resolve physical hardware (MAC) addresses to IP addresses.

On the cheap normal switch, it’s totally possible to have two subnets (say, 10.0.1.0/255.255.255.0 and 10.0.2.0/255.255.255.0) living happily in the same broadcast domain (VLAN) but each will simply ignore each other’s layer 2 broadcast traffic because the other hosts are outside the expected layer 3 subnet. This means that anyone with a network sniffer like ethereal can sniff broadcast packets and discover the existence of the other subnet within the broadcast domain. If two VLANs were used instead then nobody with a sniffer could see broadcasts from VLANs that their port isn’t a member of.

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