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linksys router setup  routing table ?

Posted on 2006-06-16
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Last Modified: 2013-12-14
Hi all  I am new here, I have a query I would like to understand. I hope I have included all relevant info:
           This is my setup
1)     I have  cable internet,

2)     My cable   modem is  provided by my isp it is   plugged into my router Linksys wrt54gs

3)     From my router I have 3 pc’s connected one   of them is wireless.

4)     Router is set to assign dynamic  ips,
5)     And they all work fine.

My question is Imho I know that my router which is the def gateway is assigning me dynamic ip addressees   that’s fine,
 Which happens  in Steps 1+2.

But steps 3 and 4 loose me, as does the routing entry table list I would really like to understand this especially in simple basic language, so an explanation would be most grateful.???

Step 1) PC1 ip  =192.168.1.100
Step 2) Linksys the def router =192.168.1.1

Step 3) IP Address:  86.14.181.189
Step 4) Default Gateway:  86.14.180.  

cable modems set up page I see this>>>>> IP address: 10.189.113.230 I guess this is called UBR.?

 Yet on my Router setup page I see this list
Login Type:  Automatic Configuration - DHCP    
      IP Address:  86.14.181.189    <<<<<<<<<<<<<    
     Subnet Mask:  255.255.252.0    
     Default Gateway:  86.14.180.1    
      DNS 1 :  194.168.8.100    
     
And I see this in my routing table entry list:

Routing Table Entry List
   
Lan and interface
Destination LAN ...  192.168.1.0              
 sub mask.............. 255.255.255.0
gateway..................0.0.0.0
 
wan interface(internet)
Destination LAN ...86.14.180.0
sub mask.............. 255.255.252.0
gateway..................0.0.0.0

wan interface(internet)
Destination LAN ...  0.0.0.0.
sub mask.............. 0.0.0.0.
gateway...............86.14.180.1
           
thank you fro your expert info:
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Question by:julietech
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Expert Comment

by:grsteed
ID: 16979256
Let me see if I can shed some light on this.  These addresses are from your ISP.

Step 3) IP Address:  86.14.181.189
Step 4) Default Gateway:  86.14.180.  

The 86.14.181.189 is the DHCP address that was given to your Router and is your "Public" address. The 86.14.180.1 (added the missing last number) is the router interface at the ISP that all your Internet traffic will go through. The subnet mask of 255.255.252.0 specifies a range of 86.14.180.1 to 86.14.183.254 for ~1020 hosts on that network. You IP may change from time to time because of the DHCP lease time but will always fall within that range.

The UBR address of 10.189.113.230 is a private address that is on the RF/Coax side of your cable modem. You need not worry about that one. It may be used to assist with DHCP on the Cable Co's network.  

The Routing table above says any traffic for the 192.168.1.0 network send out the LAN interface. Any traffic for the 86.14.180.0 network AND 0.0.0.0 (anything else) send out the WAN interface.

Hope this helps

Gary
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Author Comment

by:julietech
ID: 16979783
Gary, thanks if you just bear with me :) so is this  right the packets follow this path from my pc,,,out

1, from my Pc 192.168.1.100
2, to my router on Lan 192.168.1.1
3 here the packet is routed to Wan interface of the router 86.14.181.189 my public address(still in Lan on the Wan interface though)
4,and the packet is sent to the gateway setup for the router 86.14.180.1 the router interface at the isp,,

if this is correct ?what is 86.14.180.0 on the wan interface (internet), i have checked this it is in the routing table, and is there any utility/command  that could show this path, because routeprint or netstat do not ,, thanks  J.T.
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Expert Comment

by:grsteed
ID: 16980720
You're correct!!  That is the path of your packets from the PC out.

The 86.14.180.0 address, is part of the routing table from your router.  

> wan interface(internet)
> Destination LAN ...86.14.180.0
> sub mask.............. 255.255.252.0
> gateway..................0.0.0.0

This says that any IP address in the range of 86.14.180.1 to 86.14.183.254 (as defined by the subnet mask 255.255.252.0) will be routed out of the WAN interface with a gateway of 0.0.0.0. (see next part)

> wan interface(internet)
> Destination LAN ...  0.0.0.0.
> sub mask.............. 0.0.0.0.
> gateway...............86.14.180.1

This part says ANY address (except for 192.168.1.x addresses since they already have an exact MATCH on the LAN interface) will use the WAN interface.  This includes the 86.14.180.0 addresses that were matched in the previous part.

As far as a command to use, on Windows PC's try TRACERT <address/hostname> from the command prompt. This will show you each hop in the path. BUT it doesn't show you the Linksys hop.   ;-(  

This is due to the Linksys not responding to the ICMP packet (from TRACERT or traceroute on Linux).  This may be due to the fact the the Linksys is performing NAT (Network Address Translation) from your private network (192.168.1.x) to the Public network 86.14.180.x.  

Usually the first hop listed is your ISP gateway address, which whould match the one in your Linksys routers DHCP address.  (86.14.180.1 in your case)   Try this command and see what you get.

C\:> tracert www.google.com

With NAT, what happens when a packet leaves your PC is it has an address of 192.168.1.100 and it goes to the Linksys. The Linksys takes that packet and strips off the IP header and replaces it with a new IP header with the IP address of 86.14.181.189 and sends that to the ISP router at 86.14.180.1, which forwards it off toward the destination.

The 192.168.1.100 address is never seen on the internet because of NAT and because it belongs to a group of private address (as defined in RFC1918) that are not routed on the internet.

You're probably wondering (or you've fallen asleep) what happens with the other PC's.  Well packets from each will "share" the public IP of 86.14.181.189 on the internet, and the Linksys will keep track (using different TCP ports) which packets belong to each PC.

I hope this helps.

Cheers,

Gary




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Accepted Solution

by:
grsteed earned 175 total points
ID: 16981051
I should have clarified this a bit more. The statement "The Linksys takes that packet and strips off the IP header and replaces it with a new IP header with the IP address of 86.14.181.189"

The IP address that is getting changed is the Source address. The Destination address will stay the same.

The reply packets that come back will have a Destination address of 86.14.181.189 to get back to the Linksys.  It will then replace the IP header again to the 192.168.1.x address of the PC.

Hope this helps

Gary
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Author Comment

by:julietech
ID: 16984945
Thanks gary this has certainly explained it ,and very clearly too  now i guess i must learn more about subnetting and nat ,,
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