[RndTbl] wireless basics

Sat Jan 21 18:53:08 CST 2012

I don't mean to sound rude, but you're right: you need a good grasp on 
anatomy before doing surgery.

It sounds to me like you may have the concepts of IP address and MAC 
address backwards.

I can explain briefly (with a few over-simplifications that shouldn't 
matter in most cases):

Every Ethernet and every Wi-Fi device has a MAC address, which is not only 
unique to that device but also to that interface.  So a PC with two 
Ethernet ports and a wireless card would have three separate MAC 
addresses.
The MAC address is only used to identify a machine (more specifically, its 
interface) on the network it's connected to; they are not used elsewhere.
There is a protocol (hidden from you) that allows your computers and 
routers to automatically translate an IP address into a MAC address, but 
only for devices mutually connected to the same network.

Many "wireless routers" or "access points" simply join a wired network and 
a wireless network together so that they jointly seem to be one continuous 
network.

A router (including some wireless routers) also connects one (or more) 
local network(s) to another, foreign network.  For example, many readers 
here will have a local (home) network that connects to MTS or Shaw - a 
router is required to mediate between the two networks.
Some routers perform Address Translation ("NAT") which hides your local IP 
addresses behind a single public IP address, so that MTS or Shaw doesn't 
have to know anything about all your devices at home - they all masquerade 
as a single device connected to your ISP.
Your Linux box (probably using iptables masquerading) is probably 
performing this NAT function - and in this role, it is functioning as a 
router.  (More precisely, it IS a router.)

I'm not familiar with CentreCom, but if it was a router, it definitely had 
at least two MAC addresses.  One of those MAC addresses would have been 
visible to machines on your home network, and the other would have been 
visible to your ISP's upstream router.  There would not have been any 
reason for you to be personally aware of what these MAC addresses were.

Your base station is acting as a pure Wireless Access Point ("WAP", which 
can also mean other things), which makes wireless devices seem to be 
directly connected to the wired network.  It does not participate in IP 
routing; it has an IP address so you can more easily manage it from your 
PC.
Depending on the model, it may optionally be able to function as a router 
(like your linux box), in which case it would participate in IP routing.

IPv6 routing tables function somewhat differently from IPv4 routing 
tables, at least in terms of what you see without peeking under the hood - 
it's very common to see MAC addresses embedded in IPv6 addresses, this is 
IPv6 automatic address assignment at work and you can ignore it unless you 
want to use IPv6 for something.

You may have already deduced the answer, but no, dedicated routers look 
EXACTLY like your Linux box (to other devices on the network), and perform 
almost exactly the same functions.  The main reasons to use a dedicated 
router are (not simultaneously!): smaller size, higher performance, 
consistency of management or brand-name, proprietary routing protocols.
To make the point even clearer, the large $1M+ Cisco routers being 
deployed today actually run a version of Linux.  Much of their routing 
functionality is done with high-speed dedicated hardware, not a Linux 
kernel process, but the same functions occur in either case.
The MAC addresses on your base station are mandatory - that's part of the 
Ethernet spec and part of the WiFi spec.  Its IP address is used for 
management.

You should never see or handle MAC addresses in general everyday use; only 
network engineers see MAC addresses, and even then they're (usually) only 
used as a tool to for looking up information while performing diagnostics.

I've skipped over a lot of details and a lot of formalisms, but what I've 
said should hold true for 99.9% of home users (and probably 98%+ of 
business users for that matter).  I know I have at least 46 distinct MAC 
addresses in my basement right now, plugged into the network - and I have 
absolutely no idea what any of them are, nor do I care.  On the other 
hand, those all combined represent only 9 IP addresses and I know exactly 
what all of those are.

-Adam Thompson
 athompso at athompso.net
 (204) 291-7950 - direct
 (204) 489-6515 - fax

> -----Original Message-----
> From: roundtable-bounces at muug.mb.ca [mailto:roundtable-
> bounces at muug.mb.ca] On Behalf Of Dan Martin
> Sent: Saturday, January 21, 2012 4:20 PM
> To: MUUG Round Table
> Subject: [RndTbl] wireless basics
>
> I want to do some (hopefully minor) tinkering with my wireless
> network, but it is clear that I don't get some basic concepts.  To
> help me visualize my network at the NIC / MAC address level, could
> someone explain who has MAC addresses and why?
>
> I have a network connected (indirectly) to a cable modem.  A linux
> box serves as a gateway.  It is connected by ethernet to the WAN
> port of an airport extreme base station.  2 windows machines are
> connected to LAN ports on the base station.  1 windows machine and 2
> Macs are connected by wireless.  All the machines have manually
> assigned IP addresses, and the base station is configured in "bridge
> mode" since it appears the other options involve NAT or DHCP.
>
> My gateway box is acting as a router.  It has 2 NICs (hence 2 MAC
> addresses), 1 for inet connection and 1 to the LAN.
>
> When I used a CentreCom router, on the other hand, it appeared
> invisible, or at least I wasn't aware of any MAC addresses for the
> router.
>
> The utility for the base station lists what appears to be 2 MAC
> addresses: an "AirPort ID" and an "Ethernet ID".  I have assigned
> the base station an IP address.
>
> The Mac computer I'm using, attached by wireless, does not show the
> base station in the routing tables [except the "Ethernet ID" does
> show in the IPv6 tables].  traceroute shows only a single hop to the
> gateway.
>
> Am I to assume that dedicated routers - unlike my Linux gateway -
> appear invisible in the network and just magically connect nodes to
> each other?  If so, what are the MAC addresses on the base station
> for?  Just to access the box itself for configuration?  To access
> something connected to its USB port?
>
>
> Dan Martin
> GP Hospital Practitioner
> Computer Scientist
> ummar143 at shaw.ca
> (204) 831-1746
> answering machine always on
>
>
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