[RndTbl] wireless basics

[Sean Cody]

On 2012-01-21, at 7:22 PM, Dan Martin wrote:

> Thanks for the reply, Sean.
> 
> On 2012-01-21, at 6:21 PM, Sean Cody wrote:
> 
>> In short every Ethernet device that expects to be addressable on an ethernet will have a MAC address.  The device will then use the ARP protocol to turn MAC addresses into IP addresses (and vice versa for RARP). Then things will look as you expect.  You need a MAC to be a node on an Ethernet that is to send an receive frames for and to yourself.
> 
> ... but you don't need a MAC address to route a frame to someone else?

No.  The mail carrier doesn't need to know what is in the envelope or who send it or where it is going in order to get the mail.
The mail carrier picked up the envelope from the mailbox and brings it to the depot that puts a routing stamp on it and delivers it according to whatever process canada post uses.  Note the mail carrier just does that... you don't address your correspondence to the mail carrier but the destination.  That's the easiest metaphor I can give here.  Look up the wikipedia page on the OSI model which describes the networking encapsulation stack.
> 
>>  Cheap switches and some 'invisible' routers do not because they do not participate in the network they just act as a bridge (which is a other type of device) between two networks.  They use MAC addresses to differentiate items on the switch but don't need their own because they are not an addressable node on the network.  You don't pass traffic _to_ them but _through_ them.
>> 
>> An access point like the airport express is almost exactly the same as your Linux gateway.
> 
> The Linux gateway shows in routing tables etc.  Default traffic is directed to it via the NIC which is visible on the LAN side.
> 
> The airport extreme is completely invisible.  If I didn't see the utility showing 2 MAC addresses and an IP address (does it map to one of the MAC addresses?)  I wouldn't even know it was there.  But the entire network is connected to the gateway via the router (some wired, some wireless).

It is invisible to the tool you are using isn't showing you the detail you are expecting.  If you had some sort of wireless sniffer running you would see the 802.11a/b/g/n frames flying about.  You won't see the airport because it doesn't send out any frames of it's own, is passing along frames from other sources across it's bridge.  Depending on how it does the bridging, packet TTL's will be decremented so you can _infer_ that there may be a bridge but it doesn't broadcast it's existence from/on each frame.  Note the mail carrier metaphor above.

Don't worry about MAC addresses so much as unless you are statically assigning IPs to specific devices they won't mean much in your context.
MAC addresses do not get a one-to-one static mapping... this is a bit of rat hole so just assuming a point in time static mapping is a perfectly reasonable inference.

> 
> Does this have something to do with selecting Bridge mode?
> 
Maybe... depends on the configuration.  A bridge doesn't manipulate the packets its passing (save for decrementing the TTL in some configurations).  If the bridging device itself is assigned an IP address it is no longer transparent.

>> A router is the same.  In a wireless access point you transceive Ethernet frames into wireless ethernet frames and vice versa.  Everything operates as you expect but encapsulated in a wireless radio protocol. 
>> 
>> This whole WAN LAN is unnecessarily confusing.  A commodity router's WAN port is an uplink to your ISP.  It is still a LAN port just is expected to uplink outside.
> 
> Is the WAN port different, then?  Here the WAN port is connected to the gateway machine (private IP address).  Even though it is not 'WAN' connected, I make a point to put it 'upstream', closer to the internet.  Does it matter?  

It sure isn't.  It is an uplink by practice.  I won't get into MDI and MDI-X but lets just say that the commodity routers treat a particular port different than the rest and label it the WAN port.


-- 
Sean