[mswindowsxp] Re: Routers

  • From: Jim Betz <jimbetz@xxxxxxxxxxx>
  • To: mswindowsxp@xxxxxxxxxxxxx
  • Date: Sun, 15 Dec 2002 12:34:31 -0800

  1) A router can't protect you from a virus in an email.  Routers
     have no knowledge of the content of the packets they route and
     no capability for even looking inside the packet.  They work
     only with the IP addresses of the packets.

  A router acts as an 'IP sorting machine' and as a low-level form of
firewall (depending on the router you can block certain IP addresses,
enable or disable certain ports, etc.).  Most routers also provide
DHCP and Gateway services. 
  The firewall aspects of a router are not why you use a router.  You
use it to provide efficient routing of your IP packets - primarily to
and from the internet.  Most routers now use a technology called NAT
(Network Address Translation) which allows the router to 'hide' the
internal LAN addresses from the WWW.  In addition, most routers you
buy today - and especially the ones that are targetted to the home
environment and are combined routers and switches - also have the
capability to do your internet logon for you (the router 'dials' the
ISP and provides the userid and password to the ISP).  In order for
the router to function as a gateway it also has to either forward 
DNS requests to the ISP or provide dynamic DNS addresses to the clients
on the LAN side.
  Primarily, it is NAT that provides the firewall services.  With a
NAT router it is a LOT more difficult for a computer out on the WWW 
(aka WAN-side) to find and attack any of the computers on the LAN side 
of the router.  The port enabling aspects of a router also provide a 
small amount of firewalling - but most of the standard ports have to be 
enabled in order for the router to be used as a gateway to the WWW so 
those ports aren't 'protected'.
  NAT - what exactly is it?  The way NAT works is that it puts a
"wrapper" around the contents of a packet.  The contents of the
wrapper are only known to the router.  It works like this.  Your
LAN side computer user clicks on a link on a page on the web (think
of this is the "base" part of the request that will be sent), this
causes the browser to send a "request" to the WWW to provide the
pages of the web that that link points at, the local computer sends
it to the gateway (ie. the router), the NAT part of the router takes
the request and wraps its WAN-side address along with information on
which LAN-side computer is requesting the info around the request and
'forwards' it to the ISP (ie. the WWW), it goes thru lots of forwards
and eventually the page info starts coming back, included in that
info that comes back is the "FROM" IP address, the NAT part of the
router sees that part, strips off its 'wrapper', and sends the 'base'
part of the request to the requesting computer.  NAT stands for
"Network Address Translation" but if you think of it as a wrapping
and unwrapping service it is easier to understand how it works.
  The WWW actually 'runs' on Routers and NAT.  The majority of the
devices that are passing the packets back and forth on the WWW are 
ROUTERS running NAT.  Some of them are so complicated and powerful 
that they are more rightly called "special purpose computers". 
A lot of them are just more sophisticated and faster routers that 
aren't significantly different than what you use at home!  So when
a packet gets passed along from router to router keeps the stuff
that your NAT device/program put on it and when it gets back to
the NAT device that is 'stripped off'.  A lot of the routers that
are in the 'middle' of the chain of devices between you and the
actual website do NOT use NAT.  But you can be certain that the
closest 'outside' router to you (ie. the one at your ISP) is using
NAT technology ... with all ports enabled and with everything 
'inside' the packets coming and going just getting passed along.

wrapped into larger and larger return addresses







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