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If the ADSL modem router provides multiple wireless SSIDs how does this offer security? Is it possible to have multiple subnets with SSIDs?

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up vote 14 down vote accepted

Usually the multiple SSID option on a router leads to multiple VLANs, or some other method of separating the traffic between the two networks provided by two SSIDs. Often this is used to provide a guest network and a non-guest network, and providing some assurance that the guests cannot access the non-guest portion of the network.

Sometimes multiple SSIDs are simply there because the router has multple radios (say one at 5Ghz and one at 2.4Ghz), these are not normally separated and will not be secured from each other.

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I take it that a SSID itself cannot function akin to a VLAN? – PeanutsMonkey Nov 8 '11 at 1:38
It basically does. Traffic on different SSIDs passes over the same air but are identified for separation so that devices with different values only see each other if so configured, just as with different VLANs on a cable. However, if any device bridges the two SSIDs, the isolation goes away, just as if you bridged two VLANs. – David Schwartz Nov 8 '11 at 1:42
It basically does function like a VLAN if it does. Sorry this can't be more nailed down - an SSID is more like a switchport than a vlan. It is either part of a VLAN type isolation, or it isn't and is part of the same broadcast domain as the other SSIDs and physical ports. – Paul Nov 8 '11 at 1:49
@Kyle thanks fixed – Paul Nov 8 '11 at 1:50
and a +1 for you good sir. – Not Kyle stop stalking me Nov 8 '11 at 1:51

The purpose of multiple SSIDs is to create distinct wireless networks that can have different security properties. For example, you can create an SSID "HOME" that allows file access and printer sharing and another SSID "GUEST" that only provides Internet access. This keeps your guests from getting access to your machines.

It also allows you to change the encryption key for the "GUEST" network as often as you want (if you gave it to someone you no longer fully trust or just wish to rotate it regularly) without having to change the keys in all your permanent, trusted machines.

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Ah, you got there first. I'll bump yours. – Paul Nov 8 '11 at 1:35
@David Schwartz - Thanks. Does it mean I can segment networks into subnets? – PeanutsMonkey Nov 8 '11 at 1:37
That all depends on the capabilities of the devices. Multiple SSIDs just means there are two distinct wireless networks. What happens with their traffic is up to the devices on them. (If you have a dumb device that just bridges the two SSIDs, then you can't separate traffic at all!) – David Schwartz Nov 8 '11 at 1:43

SSID don't offer security, encryption and authentication do. I'd use the term vlan very carefully for wireless, it should not be confused with a layer 2 wired vlan. Install kismet on your old laptop, and see what you can see.

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