Time
60 hours 39 minutes
Difficulty
Beginner
CEU/CPE
40

Video Description

Wrong Subnet Mask, Wrong Gateway, Duplicate IP Address and Wrong DNS This lesson continues to cover trouble shooting common wire and switch issues and discusses the following: Wrong subnet mask: subnet indicates net portion versus host, which can throw off addressing scheme Wrong Gateway: Device sends packets for other networks, typically a router issue Duplicate IP Address: windows may show error Wrong DNS: External DNS does not recognize internal DNS

Video Transcription

00:04
So when it comes to our different devices on our network, one of the issues that can present itself with our I p address scheme is having a wrong subject mask. Now this can apply to everything from our our own device, our own client having to something wrong. Sub net mask. A particular device having a incorrect sub net mask
00:21
or even a router having a routing table with an incorrect sub net mask for certain destination network
00:27
in a wrong sub net mask throws off our entire addressing scheme. This is because we've talked about earlier. Subject masks indicate which portion of our I P address is the network portion in which portion of our I P address is the host portion.
00:42
So, for example, if we have the i p address
00:46
1 92.1 68.1 dot five
00:51
with a sub net mask of 2 55.2 55.2 55.0
00:59
now,
01:00
this is a standard classy private I P address with a standard classy private I p address subnet mask.
01:08
This is what by default are devices would think is the sub net mask for a 1 92.1 68.1 dot five address because they know that this is a classy I p address. So they would apply a classy default subnet mask to it. However, if we were using a different
01:27
addressing scheme,
01:29
we were trying to get some more clients
01:33
out of this address scheme,
01:36
and we were using the sub net mask on our network of 2 55 to 55 0.0. Then what that would mean is that
01:46
we were on the 1 92.1 68 network
01:49
with a host of our host. I d is 1.5. In our previous situation, our host would have just been 0.5
02:00
if we're trying to send packets or for using if we're sending data to devices that don't support, it's classless addressing. They don't support these changes in our sub net mask like that. Then they want they won't understand what to do with these packets. They'll be sending them to the wrong location.
02:16
If we sent a
02:19
I P packet to a destination of 1 92.1 68.1 dot five, and it passed through a device that didn't know our sub net mask or couldn't support classless addressing because it just it just went through and did default. I p addresses our I P addresses and their default class sub net mask.
02:37
Then it would assume that we meant
02:39
2 55 to 55 to 55 0
02:44
So it's important to know our it's important to make sure that we're using the correct
02:49
sub net mask when we are trying to access different devices. It's important to make sure that our different devices are able to support different are multiple lengths, sub net masks and able to send I P addresses to networks,
03:05
even if the I p address doesn't even if our I P address in our sub net mask
03:08
isn't in its default class, even if we're changing things up a little bit, doing some sub netting, we're making sure we're using devices on our network that can support that. And we're using routing protocols that support that. Remember some of our routing protocols, such as rip, if we're just using our standard r I P routing protocol standard rip
03:28
or our standard I g R P routing protocol.
03:30
Those riding protocols don't support classless addressing. So we need to remember that. And we need to take that into consideration when we're using different addressing schemes on our network,
03:40
and then we have the wrong gateway. Now our gateway. Our default gateway is going to be where our computer or where our device, since it's if it doesn't know exactly where to send them, and it needs to send them to a different network. So
03:55
it's a device, and we send this device packets for other networks. It's our default gateway when we're trying to get to the Internet would be our router that faces out toward the Internet. So far in our home, when we have five computers and a router and the Internet are default, Gateway is the router that our Internet service provider installed for us.
04:15
So
04:15
if we have the wrong default gateway,
04:18
then our computer won't be able to send packets outside of our network because our routers will only Ford packets. It will only route packets that air directly addressed to them. We can't send a broadcast packet that says, Hey, everybody, I need to send this packet out to the Internet. Can anyone help me
04:36
Our routers are going to say, Oh yeah, that's me. And then throw that packet out to the Internet.
04:41
If they did that, then think if we had multiple routers in our network, if we had multiple router set up in our network how
04:49
crazy that would be. If broadcast packets propagated throughout entire networks over routers, we'd have router sin and data all over the place. So So that's why routers on Lee respond to packets directly addressed to them. Tow Ford on to someone else so
05:08
our our routers will need to be added as our default gateway to get out to other networks. And all of our devices, which connect to a single router, which act which use one router as it's the fall gateway, are all going to be in a broadcast domain
05:25
because they're all going to be able to communicate to each other via broadcast
05:29
via that layer, too. But we just want to make sure that we aren't setting the incorrect default gateway on our devices. If we're setting them statically, we want to make sure that our that if we are setting up d h cp that we set the correct option for our default gateways in our network because we don't. Especially if our d a. C P server
05:48
is not on our default gateway. If we set up an actual d d h c P server,
05:54
we'll need to enter in the default gateway code so that when it hands out, I p addresses and it hands out sub net masks and hands out D. N s addresses. It also hands out the address for those devices Default gateway,
06:05
And then we have duplicate I P addresses. Now, when we we have duplicate on I p addresses in the same network, that's obviously very bad. It would be like having two houses in the same in the same city with the exact same street address. Be very difficult to manage because
06:24
everything else about them is the same. They're on the same. They're in the same
06:28
state. They're in the same zip code. They're in the same city, so we can't have them having the same street address and house address the same thing with our packets. If our clients, if we have two devices on the same network with the same private I P address than those two devices. We don't really know which one to send it to because we're sending
06:47
ah packet destined for this I p address. But that I p address could be this person or this person.
06:53
Now, this is not referring to multicasting. Multicasting is where we have devices where we send packets to a multi cast address, and those packets are then sent on to all of the members of that multicast address. This is talking about using a standard private I p address range of standard addresses
07:11
such as the 1 92.1 68.1 dot five address,
07:15
and then that address is inadvertently assigned to multiple computers.
07:18
Now Windows will typically give us an error that says, there's an I P address conflict on your network and what we can, and that'll give us a hint that we need to resolve it. But we need We may need to find that information out manually and may meet. You may notice that we're experiencing network issues, so we'll need to track down and see if there's any duplicate I P addresses out there.
07:38
So
07:39
when duplicate I P addresses maybe maybe inadvertently caused by two devices having the same static I p address set on and that will cause I p address conflicts. And we may also have i p address duplicate i p addresses caused by D H cp servers that are set up incorrectly
07:59
if we set up
08:00
a single device tohave a static i p address and then say we turn that device off line for a couple of days or a week or two and then we never set up a d h cp reservation for that device and that I, p addresses in r GHT p scope than on th e p server may hand that same address out to somebody else. Then the next time we turn on our statically addressed
08:18
device
08:20
are statically address device isn't necessarily gonna go out and look for an address that the HDP hasn't given out yet. So there's gonna be an un i. P address conflict between our device R i p R device with the static i p in the device that received an I P address from the D. A. C P server. That's why it's a good practice, too.
08:39
Sets that set a certainty HCP scope
08:43
and then set static addresses that are outside that scope and also set reservations for those addresses even though they're outside our scope of addresses that were handing out
08:52
and then, lastly, our D A c P servers. We may have more than one on our network if we have more than one. D H C P server on a single autonomous network will need to make sure that those D H CP servers have different scopes of addresses that they hand out. If those D A C P servers have the same address scope or they have overlapping address scopes,
09:11
they may start to hand out the same address the different
09:15
different clients, thus also resulting in duplicate I p addresses. So check out your static, check out your static settings on your clients, and also check out your D H CP servers to make sure that there aren't and she's going on there.
09:28
And then we have the wrong the wrong D. N s settings. Now, DNA stands for our domain name resolution, and our domain name resolution is going to be what resolves fully qualified domain names such as www dot google dot com or
09:43
file server one dot cyber Eri I t dot local to an actual I p. Address that weaken network to sew this d n s D. N s settings are very important. We have d. N s server set up internally, and they're also external being s servers out there on the Internet
10:00
that we may set as our d n a server in order to connect two different devices.
10:03
Now,
10:05
using a D. N s an external being s server, a certain de NS server out on the Internet as our primary D. N s server will allow us to connect to resource Is that air out on the Internet? It will allow us to connect to out toe websites and dot org's and dot coms. But what it won't do is
10:22
those external being s servers won't give us information for devices inside our network
10:28
because we don't have the authority to put that information on there. And if we did, it would just be crazy trying to connect to an actual public D. N s server because it would be constantly changing their information on there.
10:37
So external dina servers don't know about our internal devices, and this is also good for us because our security we don't want other people being able to enumerate our internal devices.
10:48
So
10:50
if we have internal file servers and internal, indifferent internal FTP servers, and if different internal devices that we may need to connect Thio and we have a set external D N s, then we'll be able to navigate the Internet. But we won't be able to get to our file server because we will ask
11:07
W we'll ask whatever the NS server we have,
11:11
we'll ask Google's D. N s server eight dot a dot a 8.0.0.8 will say, Hey, Google, do you know where file server one dots I bury, I t dot local is and Google Google's The NF server will say I don't know what you're talking about. That local isn't a local isn't a qualified route domain that you can use. Get out of here
11:30
so it won't be able to give us the I p address that we need. That's why we'll need to set up internal being s servers that do have that information on them. We set up internal D N s servers where we manually insert. Okay, This this name equals this I p address. This name equals this I p address and those being s servers may work
11:48
toe automatically update that information as well.
11:52
So if there's i p addresses, change will still be able to resolve them.
11:54
But those internal D. N s servers we don't want to have to constantly maintain with all the i p addresses of everywhere on the Internet, that would be ridiculous. So what we do is we set up the NS forwarders. What a D N s forward or does is it is the next level being s where our D. N s checks if it doesn't, no one answer.
12:15
So if
12:16
we go to our local being s and say hey, local D N s, where is sai berry dot i t Our local being s says I've never looked that up before. I don't have a cash for it, so I don't know, let me check with Google or let me check with Verizon's or Comcast D. N s server. It goes out to them
12:33
and that being S server either finds an answer or
12:37
either finds out where that is, or it returns back its answer that it knows to us and then that external, the N s server tells our internal being s server. Oh, hey, here's where cyberia dot i t is. And then when that d. N s server, then when our internal dina server has that information, it may cash it
12:56
and for later use if necessary
12:58
and then passes on the i p address to us, which we also cash.
13:03
So D and D. N s information is very important when we're connecting out to the Internet and we're more connecting to our local devices. So don't just set a local, are set an external D. N s and didn't just forget it. If you have internal devices that you may need to connect to and may need to resolve, you'll need to set up internal D. N s structure.
13:22
So our d A C P options also include a D. N s option. So we want to make sure that we if we have an internal being s server, that we have the correct D H cp options that indicate when our clients are pulling automatically their I P address
13:37
There are also pulling d. N s information. They're also pulling the correct internal bien s
13:41
so make sure that we have those D n s that d n s set up correctly. If we do notice that we are having issues connecting out to certain Web pages. For example, if we try to type in www dot target dot com and we aren't getting the Web page or we're getting a Web page that looks kind of weird,
14:00
then we could test and see if it's our d. N s
14:03
by saying, OK, let me result. Let me go on a Let me find out what Google target dot coms I p addresses. We get that I p address and then just navigate to that I p address
14:16
in our Web browser, and then it brings up the Web page just fine. Then we know there's an issue with our d. N s because our d. N s resolves there's fully qualified domain names to that I p address. And if it's giving us an incorrect I P address or not getting the right information, then that's when we're having issues. But if we can result, if we can go to a certain page or we can go to a certain client
14:35
using its I p address, but not its fully qualified domain name that we know that there's probably an issue going on with the N s.
14:41
So keep that in mind, Keep a track, keep track of your D. N s, and it will help you with when you're trying to navigate when you're trying to resolve different names on your network.
14:52
So thank you for joining us here today on cyber dot i t. Today we talked about how to troubleshoot common issues that we may experience with different routers and switches. So we talked about everything from checking our D. N s settings to checking our ports endings and checking out if we have any bad modules or bad cables. So
15:09
hopefully we'll be able to use this information,
15:11
troubleshoot your own network settings, troubleshoot your own switches and routers a little bit better, and hopefully you'll be able Thio see us here next time and see our next video here on cyber dot i t

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Anthony Harris
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