[In Progress] Quick guide to troubleshoot your network problems

[mrenusso]

Networking problems can be a big pain to resolve. I've been doing it for quite a while and I'm usually the guy my friends call when they're in trouble. So I figured, might as well help all of you guys out there !
If you have any questions or remarks, do not hesitate to comment below. l will try to keep it pretty simple for everybody but hit the Networking section if you have tried everything and still can't figure out what's wrong; I'll be glad to help.
So here's my quick little guide to help you troubleshoot your network problems by yourself, like a boss.



0. The OSI Model

Before we dive into troubleshooting, there's a few little theoretical things you need to understand.
When developing network technologies, engineers needed to create a theoretical frame structure for all things networking.
They divided the network structure into 7 layers creating The OSI Model.


Source : about.com

As the above picture shows, this is pretty theoretical but it has its perks for troubleshooting. Layers are numbered 1 to 7 starting at the bottom (Layer 1 : Physical) to the top (Layer 7 : Application). You'll also notice in blue the devices in each layer, that'll help you in the future.
Climbing up the layers is the "proper" way of troubleshooting network problems and will save you a lot of time if done properly.
Also, and before anything else, RESTART ALL DEVICES ONCE BEFORE CHANGING ANYTHING, it solves an enormous amount of problems.
So let's jump in and start with the physical layer.

Quick tip : To remember the Layers, this sentence helps by containing words starting with the first letter of each layer.
"All People Seem To Need Data Processing"


1. The Physical Layer

As its name stands, the physical layer contains all things physical. I'll try to keep it as simple as possible because this is my nitty-gritty, haha.

It ranges from the copper pair of wires for your xDSL connection, the ethernet cable between your switch and your PC, the air (and all the electromagnetic waves it contains) in which your WiFi connection go through or the fiber optic cable running from the Telephone Exchange to your house.



So before you try anything on the configuration of your PC, your router or your switch, make sure the cables are working properly (I use a tool similar to this). In the case of WiFi, testing is more difficult since testing equipment is über-expensive. But a quick way to try is to test with another WiFi device and see if it works.

Once you made sure your cables are working properly, we can go up one layer and begin Data Link Layer testing.

Most layer 1 problems are easy to troubleshoot in a LAN environment. It gets really tricky in WANs with xDSL or cable because the amount of factors to take into consideration is much higher. So don't yell at your ISP if they can't figure out what's wrong with your Internet, it can take months to figure out what is causing random disconnections.
I remember a few years ago, I was auditing a client for random disconnections. We searched for weeks until we found out that his neighbor TV decoder adapter was leaking current all over the place and was perturbating the ADSL connection. Pretty hard to find, eh?!


2. The Data Link Layer

The data link layer is the first level of electronics. Its role is - but not limited - to convert the electrical signal received on the wire (or the electromagnetic waves for WiFi) into bits (0s and 1s) and vice versa. Problems at the 2nd layer are rare but can happen.

 

A Cisco Network Interface Card.

 

Most common point of failure is the Network Interface Card. Try using a different NIC and see if it works.
On your switch (or All-In-One modem for most people), try using a different port. A quick way to determine if a NIC is working is by looking at the LED on the port. If your device stays dark, then it's probably failing.

For WiFi, make sur your Access Point is turned on (sounds stupid but happen more often than you think). Also, try to see if other devices can connect to your Access Point (or your All-In-One Modem again). If other devices can connect successfully, try connecting your device to another Access Point. In case of failure, your NIC might the problem. Change it and see if the problems still occurs.

Now that you have eliminated all suspicions at the Data Link Layer, let's climp up another layer !

Another problem which could be occuring at layer 2 is incorrect VLAN setups. Usually reserved for Enterprise grade equipment, VLAN problems are easy to troubleshoot. Just log in your managed switch, run a "show interfaces" and make sure your VLANs are setup correctly.
Finally, MAC specific problems can also happen such as incorrect MTU on one or both side, incorrect duplex configuration, etc.But let's keep it simple, shall we?


3. The Network Layer

This is where most of the problems happen. And this is where it's the most complicated to isolate.
The Network Layer is where the IP addressing takes place. And by IP addressing, I mean all things IP, from IP addresses of devices to gateway configuration, etc.
So in order to keep things as simple as possible, please follow the list below, it's how I troubleshoot IP problems on a daily basis.


1. Check your network configuration by running the following command in a terminal or command prompt
 

For Windows users : ipconfig
For Linux (and OSX I believe) users : ifconfig

This will show your current network configuration on all your network cards. If the NIC you're trying to connect with has a 169.254.x.x address (fe80::/10 for IPv6) then it means your card didn't receive its IP configuration from your router.

In that case, run the following commands :

For Windows users : ipconfig /release

ipconfig /renew

For Linux users : sudo dhclient -r

sudo dhclient

For OSX users, it's more complicated so just logoff and log back in.

Otherwise, most routers have something called DHCP enabled (Log to your router Web Interface to make sure it's activated). Dynamic Host Configuration Protocol is a really useful protocol where the DHCP server (your router most of the time) sends all the network configuration to the devices connected to its network.

Credit : http://www.tafenetworking.net/

In 90% of cases for consumer LANs, your IP address should look like 192.168.0.x or 192.168.1.x and your network mask should be 255.255.255.0

If your address looks like above, you can try pinging (network terminology for talking) your router by running the following command in a terminal or command prompt :

For all OS : ping [your router address]

where [your router address] is typically your address with .1 or .254 in the last byte. For example, my IP address is 192.168.1.27, my router is 192.168.1.1

Once you can ping your router successfully, you may try pinging other devices on your network to be sure.

Now, we know that you can communicate inside your LAN since you're pinging your router and other devices inside the same LAN. Great first step !

2. Make sure you can communicate outside your network

So we know we can communicate inside our local network, but what about the rest of the world ?

You see, when you are trying to communicate outside your LAN, it gets a tad more complicated. We'll just scratch the surface, don't worry ;)

First thing you'll need is a default gateway.

 

Credit : http://howtoforge.com

 

A gateway is responsible for forwarding data that comes from internet to your PC and vice versa. Normally, if DHCP is working (and it should, as you verified above) you should be provided with a gateway address which is your router address. To check, simply run the following commands :

For Windows users : ipconfig
For Linux (and OSX I believe) users : ifconfig

You should see your router address next to Default Gateway.

If you don't see anything next to the Default Gateway field, then post in the Networking section, it'll take forever to cover all the cases here.

Now to test that your gateway is working properly, try pinging Google's DNS servers by running the following command :

For all OS : ping 8.8.8.8

If the ping succeed, then your gateway is working properly, nice !
You can talk with the rest of the world, but there's one little thing we need to verify if you still can't connect to your favorite website.
 

3. Domain Name Resolution : How to change http://linustechtips.com into an IP Address ?

As you may have already understood, interconnected devices communicate with each other using addresses. The problem is to remember all those addresses. For example, it's way easier for a human being to remember http://google.com than 173.194.112.95, right ? As such, DNS was invented. Its role is to change an intelligible address such as http://linustechtips.com into a network address like 123.134.56.21.

The principle behind it is very simple : Servers across the globe store in enormous databases intelligible addresses and link them with IP addresses. So everytime you try to connect to your computer sends a request to a DNS server asking for the corresponding IP address.

Credit : http://www.howstuffworks.com

[mrenusso]

reserved for future use.

[mrenusso]

reserved for future use.

[mrenusso]

reserved for future use.

[mrenusso]

reserved for future use.

[loepa]

I've been waiting for this kind of guide.

 

By the way.

Can you fix your first post?

Try using full editor or something (I forgot the name) It should fix the problem.

[mrenusso]

Fixed ;)

[3DDude]

Yes i remember the OSI model:

 

All People Seem To Need Data Processing sentence.. :)

 

Layers 1 to 3 are required to well lets say: "stay or get connected."
The rest application and how the data presents itself.

 

Nice guide :)

The only comment i have is you say

1. OSI model

2. Physical

3. Datalink

4. Network

--> maybe change that numbering because people might think that layer 2 == physical now? :P

*so remove the 1. from OSI model ( just dont put a number before that) and start 1 @ Physical :)

[mrenusso]

Yes i remember the OSI model:

 

All People Seem To Need Data Processing sentence.. :)

 

Layers 1 to 3 are required to well lets say: "stay or get connected."

The rest application and how the data presents itself.

 

Nice guide :)

The only comment i have is you say

1. OSI model

2. Physical

3. Datalink

4. Network

--> maybe change that numbering because people might think that layer 2 == physical now? :P

*so remove the 1. from OSI model ( just dont put a number before that) and start 1 @ Physical :)

 

Confusing indeed for the beginner, let me change that right now, thanks for the input !

I remember bi***ing about the OSI model during my studies, but damn is it useful after all. Like I use it all the time.

[looney]

looking good man!

[loepa]

Maybe a little off topic. But I keep getting this problem with my modem and router.

 

Where I can't access my modem and router by wi-fi and if I use Lan I can't access it either (by typing 192.168.1.1 to access it) and I can't connect to the internet.. But all the light with my modem and router is normal.

 

My solution atm is by turning on and off my modem and router.

 

My modem is 1 weeks old and my router is 1 year old they both have the same brand (TP-Link). This problem also occur with my old modem which I recently replace with the new one.

[mrenusso]

Maybe a little off topic. But I keep getting this problem with my modem and router.

 

Where I can't access my modem and router by wi-fi and if I use Lan I can't access it either (by typing 192.168.1.1 to access it) and I can't connect to the internet.. But all the light with my modem and router is normal.

 

My solution atm is by turning on and off my modem and router.

 

My modem is 1 weeks old and my router is 1 year old they both have the same brand (TP-Link). This problem also occur with my old modem which I recently replace with the new one.

Wait, so you're trying to connect to your modem or router interface and it only works after you reboot them ?

[loepa]

Wait, so you're trying to connect to your modem or router interface and it only works after you reboot them ?

 

Well let me explain this in depth.

 

Let's say I'm using my internet. Both my modem and router is ok, no problem at all. But all of a sudden, out of the blue my internet dies. But my adsl is still on. All my cable are ok and connected. BUT I CAN'T access my router and modem at all in any way.

 

If that happens all I can do is reboot both of them. After I reboot them I can use the internet and access both of them like nothing happenned.

 

I hope this explain everything better. English is not my main language sorry for bad grammar and such.

[mrenusso]

Oh right, gotcha ! If all the LEDs on your modem are green and the same as usual, I suppose your modem is working fine all the time. When it happens, you should try to connect directly to your modem and see if you can connect to the internet. 

I'm leaning towards a router problem more than a modem problem. 

So next time it happens, try directly plugged into your modem and also try to reboot your router only.

You should probably create a new thread and keep us posted !

[loepa]

Oh right, gotcha ! If all the LEDs on your modem are green and the same as usual, I suppose your modem is working fine all the time. When it happens, you should try to connect directly to your modem and see if you can connect to the internet. 

I'm leaning towards a router problem more than a modem problem. 

So next time it happens, try directly plugged into your modem and also try to reboot your router only.

You should probably create a new thread and keep us posted !

 

Yeah I made a new thread in troubleshooting sub-forum.

I gave a bit more explaination there.

http://linustechtips.com/main/topic/35869-modem-and-router-problem/

[I'm Batman]

stop spamming

[3DDude]

By the way, you did seem to forget layer 8 of the OSI model..

(enduser.. ) :P :D :rolleyes:

[Blade of Grass]

No updates? 

I think that non-network types will find this useful, update it!