cable connections

Posted on 2006-07-09
Medium Priority
Last Modified: 2013-12-14

recently we acquired corporate  ADSL connection to our company.

i configured a pc as firewall and router.
one  network interface of th pc is labelled as wan and public ip was configured.

the other end has ip and labelled as lan interface  and connects to core switch.we have a dhcp server for lan purpose.

the problem started when tryiong to connect internet by directly connecting cat  5e ethernet  straight cable from adsl modem to the wan interface of the pc.the firewall pc and clients cannot connect to internet.

suspecting the problem could be the cable ,it was checked with cable tester but it is perfect. even i tried with another  stright cable but the same problem exists.

i  alos tried with a crossed cable connecting the modem and pc directly,but no use.

amazingly the pc's can connect internet only when the adsl modem is connected to an ethernet switch and the wan  interface of the pc  also connected to the same switch.

so please can u figure out what actully went wrong....

Question by:ganitham
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LVL 14

Expert Comment

ID: 17069658
sounds like the crossover cable is bad.  Make sure that it is actually a crossover cable.

check out this url if you are intending to make your own.  A prefabed cross over is guaranteed to work.


LVL 57

Expert Comment

ID: 17070269
Do you have:

 ADSL <-- CAT5 --> NIC1 PC NIC2 <-- Core Switch-->

If so, then straight cables should be fine, no need for cross over cables.  I am assuming that NIC2 is on the same IP subnet as the rest of your network and that this is the IP address used as the default router for your network.  

Does NIC1 get an IP address assigned to it from the ADSL side?  

Can you access the Internet from the PC you configured as a router?  

Does the PC you configured as a router have a firewall running on it?

Author Comment

ID: 17070850
yes the subnet of NIC2 ( is same as that of rest of network (

the ip given are public ip's. and they are manually configured.

the pc configured as router also servers as firewall.

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LVL 57

Expert Comment

ID: 17073369
I would double check your firewall configuration.

Author Comment

ID: 17074968
but i have mentioned that it is now working when both the adsl modem and the wan interface of pc  connected to a switch.
ADSL<--CAT5--Switch--CAT5-->NIC1 PC NIC2 <--CORE Switch-->

but it does'nt work when the pc is connected directly with adsl modem .
ADSL <-- CAT5 --> NIC1 PC NIC2 <-- Core Switch-->

LVL 57

Accepted Solution

giltjr earned 500 total points
ID: 17076046
Ah I misunderstood, I though you stated that :

ADSL<---- >CORE Switch <-- network -->

was working (that is no PC invloved).  What the following, the 1st setup you had working?

     ADSL<--CAT5--Switch--CAT5-->NIC1 PC NIC2 <--CORE Switch-->

What type of swtich did you have between the ADSL and the NIC1 on the PC?  You may want to connect as:

     ADSL<--CAT5--Switch--CAT5-->NIC1 PC NIC2 <--CORE Switch-->

Go into the managment interface of the ADSL router and "release" the connection, then reconnect as:

     ADSL <-- CAT5 --> NIC1 PC NIC2 <-- Core Switch-->

If your first attempt was with a swtich between the ADSL modem and the PC, depending on the type of switch it was, the ADSL modem may have seen the switch MAC address and is not allowing anything but that MAC address to talk to it.

When you have:

     ADSL <-- CAT5 --> NIC1 PC NIC2 <-- Core Switch-->

Does the ADSL modem show that the NIC interface is active?  Does the PC show that the NIC interface is active?  Can you ping the ADSL modem from the PC?
LVL 27

Assisted Solution

pseudocyber earned 500 total points
ID: 17080372
Assuming none of your ports (modem, switch, or NIC) are Autoconnect or Auto MDI/X, then they're both Data Terminal Equipment (DTE) - the modem and the pc.  So, therefore, you need a cross over cable to connect them.

If you used a cross over and it didn't work, get another crossover - a known, good, working crossover cable.  

Holding both ends in your hand, looking down on the pins towards you, on the left on one end should be white green and green.  The other should be white orange and orange.

Author Comment

ID: 17083267
as the problem i faced is with one the client,i need time to check with the cross over cable  and said by pseudocyber .

and also retest the conenctivity according to  glitjr.

i will inform asap.
LVL 14

Expert Comment

ID: 17083830
I kinda figured it may have been the crossover cable...
LVL 23

Expert Comment

ID: 17087595
hi there

what is the gateway for the lan clients have u configured the DHCP to apart from providing the ip address to clients assign the default gateway.   Ideally speaking this would have been already done, but just trying to confirm.

when ur connecting the cat5 cable running from the adsl modem are u connecting to the uplink port of switch or one of the normal ports.

When ur connecting the cat5 cable to NIC1 of PC where are u connecting the cat5 running from NIC2 to a normal port or an uplink port.

I believe that ur are missing out in this which is causing the issue. or the default gateway issue.

pls revert back for clarifications


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