Need some advice on learning networking

[BillyCarpenter]

Quick note on the ping command.....


Disclaimer: This isn't part of the course that I'm taking. I took the time to go outside of the course to learn this because of the tricky wording that is used in the CCNA test. It was confusing me and I needed to be clear on what they were asking.


The ping command can get down in the weeds if you want to go there, but here's what you really need to know for the CCNA test.

* Ping falls under the ICMP Protocol

** ICMP is defined in RFC (Request For Comments) RFC792. Looking at its position in the OSI model we can see that it's sitting in the Network layer (layer 3) alongside IP. There are no ports used with ICMP, this is because of where the protocol sits in the OSI model. Ports are only used for protocols which work at the Session layer and above:

*** Ping is also known as "echo request" and is identified by the number 8 if you're capturing a ping packet on Wire Shark. If you're capturing a echo (ping) reply on Wire Shark, it will be identified by the number 0.

Ping Request: Identifier number = 8

Ping Reply: Identifier number = 0



There's a lot more to it but I don't necessarily know that we need to know all of the information. Again, I'm doing this outside of the framework of the CCNA course.

[BillyCarpenter]

I'm gonna wrap up the discussion on "the life of a packet" by ending with this.

- If you're studying for your CCNA and put the time in on tracing a packet from one end of the network to the other, you shouldn't have much of a problem with the exam questions. Just pay close attention to the wording because they'll try to trick ya!!!


A couple of other thing to be aware of.


1. If you see 2 routers linked together by a serial cable, please be advised that this is a point-to-point connection and doesn't care about a MAC ADDRESS. If they ask for the mac address at that point in the network...there is none.


2. Pay attention if they're asking what you'd expect to see on a ping request vs. reply. These are 2 different things and the mac addresses are usually reversed.


Lastly, spend some time on this or I think you'll regret it on test day.

[slimslob]

In easy to understand language, the PC knows it needs to connect to the default gateway only when it's trying to send a packet to a different network. Bascially what the PC does is compare the destination IP address with it's own address and if it's on a different subnet it knows to contact the default gateway.

There's only one small problem. The PC doesn't know the MAC Address of the default gateway. How does the PC find the MAC address of the default gateway? Hhhmmmm.....

But the PC might know the IP Address of the default gateway form the information it get from the DHCP or in the absence of DHCP from what was programmed for the adapter. In fact if the PC does not have the IP Address it might not be able to access anything that is not within its subnet.

And then there is the possibility that the default gateway might not be the gateway it needs to use. I used to work on the dictation system at a local hospital group. They had 3 hospitals in Bakersfield. In addition to the default that gave each hospital their own internet access there were 3 leased lines giving each hospital direct access to each of the other 2.

[BillyCarpenter]

But the PC might know the IP Address of the default gateway form the information it get from the DHCP or in the absence of DHCP from what was programmed for the adapter. In fact if the PC does not have the IP Address it might not be able to access anything that is not within its subnet.

And then there is the possibility that the default gateway might not be the gateway it needs to use. I used to work on the dictation system at a local hospital group. They had 3 hospitals in Bakersfield. In addition to the default that gave each hospital their own internet access there were 3 leased lines giving each hospital direct access to each of the other 2.

You're a little off base. At least from my understanding.

Your PC will always know your default gateway IP address because we are the ones that enter the default gateway into the PC or it's done by DHCP like you said. However. our PC doesn't communicate with the default gateway using an IP address. That is done at layer 2, which means that the PC must learn the Mac Address for the default gateway.

As far as the scenario that you gave for the hospital, I'm gonna have to think on that one.



Remember that each interface on a router represents a different network and that each interface has its own Mac Address.

[slimslob]

You're a little off base. At least from my understanding.

Your PC will always know your default gateway IP address because we are the ones that enter the default gateway into the PC or it's done by DHCP like you said. However. our PC doesn't communicate with the default gateway using an IP address. That is done at layer 2, which means that the PC must learn the Mac Address for the default gateway.

As far as the scenario that you gave for the hospital, I'm gonna have to think on that one.



Remember that each interface on a router represents a different network and that each interface has its own Mac Address.

For a leased line, think in terms of 2 routers connect serially

[BillyCarpenter]

I have a question for the board. I just learned how to use telnet to see if a port is open. I was pretty excited to learn how to do it.

Here's the question. Is this something that is normally done in the real world? Is there a better/easier way to see if a port is open?

[BillyCarpenter]

Well, Fedex will be delivering my Dell R620 Server today along with (6) 900GB SAS Drives. I also ordered some RAM but I forget how much. All of it will be delivered today. I'm excited.

[rthonpm]

I have a question for the board. I just learned how to use telnet to see if a port is open. I was pretty excited to learn how to do it.

Here's the question. Is this something that is normally done in the real world? Is there a better/easier way to see if a port is open?

Because telnet has no security, you'll find it's disabled in a lot of environments. Windows hasn't had a telnet client installed by default since probably Vista, and most Linux distros don't install it by default either. I keep a telnet client on a Debian VM for testing and for a couple of old appliances that still use it, but I don't use it that often. If you have access to the machine you want to query, netstat is a much better tool. In the Windows world, PowerShell has the cmdlet Test-NetConnection that will let you query a remote computer. In Linux, nmap will do the same type of thing.

[BillyCarpenter]

Because telnet has no security, you'll find it's disabled in a lot of environments. Windows hasn't had a telnet client installed by default since probably Vista, and most Linux distros don't install it by default either. I keep a telnet client on a Debian VM for testing and for a couple of old appliances that still use it, but I don't use it that often. If you have access to the machine you want to query, netstat is a much better tool. In the Windows world, PowerShell has the cmdlet Test-NetConnection that will let you query a remote computer. In Linux, nmap will do the same type of thing.

Thanks, rthonpm. That's very helpful.

[slimslob]

I have a question for the board. I just learned how to use telnet to see if a port is open. I was pretty excited to learn how to do it.

Here's the question. Is this something that is normally done in the real world? Is there a better/easier way to see if a port is open?

If you are only wanting to see if you have any ports open to the rest of the world, you can't beat Steve Gibson's Shields Up. Some of his other utilities are also useful. GRC | Gibson Research Corporation Home Page