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Hey everyone and welcome back. So what we're going to be talking about and this skill right

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here is all around the concept of DHCP. Now DHCP stands for the Dynamic Host Configuration

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Protocol and it is not an exaggeration to say that DHCP is one of the most useful protocols

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in all of computer networking. So I suppose it does beg the question what is DHCP, why

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is it useful for us and what exactly do we need to know about this protocol to meet the

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criteria set about within the LPIC2 exam objectives. Well the good news is that that is what we're

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going to be digging into within this very skill. So the very first thing I should do I suppose

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is tell you what DHCP does and why we need it. So the first thing is if you can imagine

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that you happen to have a fairly large network. Let's maybe say this was some type of corporation

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or maybe perhaps you were administering the network for a university campus. Imagine you

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had thousands of people who needed to access thousands of computers. Now we understand

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that the very basics of computers rely upon IP addressing in order to actually communicate

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with one another. Now have you ever wondered how do we actually manage the IP addressing,

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i.e. how do we decide which IP address goes to which computer and when we're dealing with

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a lot of computers how is this even manageable because think about it. Imagine here that we

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had some type of network switch and let's just imagine we had a computer here and here

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and here and here and we just keep adding computers all around the network. Can you

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imagine how much fun it would be to have to run around the campus going across all the

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different floors or all the different floors within your company and sitting down at each

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desk and logging into each computer and manually typing in the IP address information for each

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of these clients. You can imagine that this is going to become pretty cumbersome and pretty

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laborious and inefficient really quite quickly. Not only that, imagine the headache of trying

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to keep the IP addressing straight in your head. Let's say maybe this device right here,

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you gave it the IP address of 192.168.1.49. You have to make sure that you don't inadvertently

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give say for example this device or for what it's worth any other device on the network

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that same IP address because then we would have IP address duplication and this is going to cause

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a lot of issues with respect to communication within our network. So as it stands, been able to

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manually configure each device statically whilst it is possible it is not desirable nor is it

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efficient and the good news is is that realistically we don't have to do such a thing in the real world.

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That's because the real world gets to enjoy all the glories of DHCP which is going to handle this

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process for us. So what DHCP is, it's kind of in the name. This is the dynamic host configuration

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protocol i.e for all of the hosts within our network they can dynamically be assigned their IP

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address information and the way this is done is via something called a DHCP server. Now what a DHCP

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server is going to do, it's going to ultimately be configured to have things such as DHCP pools.

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Now what a DHCP pool is, it's ultimately a range of addresses. So you could imagine if we were

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administering let's say the network of 192.168.0.0 slash 24 again if you remember you're subnetting

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from LPIC 1. This means that we can have around 254 computers using IP addresses within this

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particular address space. That is allowing for the network address and the broadcast address of

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course. So even in this relatively small network having to do the configuration 250 odd times is

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always a bad idea. Like I say with DHCP we can have this assigned automatically. All we do is

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we specify the range of particular IP addresses we want to give. So let's say we wanted to give out

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an IP address between 192.168.0.2 all the way to 192.168.0.254. We could do such a thing. This could

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be our range. Now you might be thinking why would we not start with .1 for example here. Well think

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about this all these devices on this particular network would need to have a default gateway

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so that they can actually reach remote networks i.e other networks or things like the internet. So

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maybe it's an idea that we actually reserve an IP address and in this case here I've taken the very

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first available address 192.168.0.1 and we will just configure that on our router. So we don't want

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that address to be handed out to any of our clients that is going to be outside the range. So within

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our pool we can specify which IP addresses we want to be available to our potential clients. Now this

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is not the only thing that we can hand out with DHCP. In fact that router IP address I just specified

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there we can also pass that on to our clients. Pretty much saying we can give a client an IP if

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they happen to request it. We will also give them their subnet mask which again if you recall from

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lpic 1 we talked about the subnet mask which is going to allow us to separate host and network

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based information and also we can actually hand out that default gateway information to a host so

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that when a host happens to make that request they get the IP, they get the mask and they also know

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which address they should send to when they want to reach a remote network and not only that still

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we can also use DHCP to hand out DNS information so we can just specify this is the DNS server

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you want to use. So now let's imagine the same scenario this time with DHCP then. So let's say

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we have our little network switch and we have all of these clients within the network. Now I'm not

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going to draw quite as many clients but you can get the drift. So these are all connected to the

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switch here okay but within the same network segment here imagine that we have this DHCP server.

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Now what I will say is that the DHCP server does not actually have to be on the same network right

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here i.e. on the same network segment we can actually have DHCP servers on remote networks

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but we'll actually talk about that when we talk about DHCP relays very very shortly. Now imagine

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this device right here suddenly comes online it doesn't have an IP address and in fact let me just

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draw a router on our network which can be our default gateway. So this device here suddenly

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comes online and it needs an IP address so what it does it's going to send a broadcast message.

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Now you may recall how broadcasts operate from the LPIC1 certification this is ultimately a message

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that is going to be communicated to all of the other devices within our network. So this broadcast

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is going to go here into the switch and then it's going to be directed out here and out here

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and out here and here and here and here. Now this broadcast message is going to have a very specific

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name this is going to be known as a DHCP discover message. This is ultimately probing other devices

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on the network trying to discover if any of them happen to be a DHCP server. So naturally all of

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these ones here do not respond they just ultimately ignore this broadcast message but here the DHCP

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server does respond. So the DHCP server says hey I'm a DHCP server how about I offer you an IP

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address okay so from within the pool configured on the DHCP server an offer is made this is the

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second step within the process. So this device here gets an offer of let's just maybe say the IP

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address offered was 192.168.0.10 and this device says hey that's good enough for me I would like to

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take that offer. So what happens and by the way I suppose I should actually mention here that the DHCP

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offer is not sent as a broadcast to all the devices on the network it is a directed unicast

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message just for this particular device targeting that particular device's MAC address. Now like I

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say if this device here wants to take that offer it's going to send a request. So this device responds

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with its own unicast message now directed to the DHCP server as a request and the request is pretty

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much saying yes I would like to request that you actually do give me that IP address configuration

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and finally the server will acknowledge this request and says okay that sounds good to me you

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can have that IP address so we have that acknowledgement at the end. Now it should be noted is that

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within that very basic transaction I did there all that was offered by the server was an IP

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address but that was simplified in real terms that offer could contain IP address information as well

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as default gateway information i.e. it could also be providing the IP address for that router that

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was on the network and again it could also be providing the DNS information. So as a recap the

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transaction was as follows we have this nice little acronym that we can use to memorize this this is

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called DORA. So we first have our discover message which happens to be a broadcast to all devices

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on the network segment trying to find a DHCP server. Once a DHCP server on the network receives

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this discover message it's going to respond with an offer from its pool offering up the IP address

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configuration to that particular client. If the client likes and wants to receive those configurations

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the client will then send a request as a direct unicast now to that DHCP server and as the client

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has asked to receive that information the server will acknowledge it with a direct unicast message

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and that really is the basic process. Now this process is very useful for understanding when

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you want to be looking at DHCP logs to understand what exactly is going on so it definitely is

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worthwhile to familiarize yourself with this exact process. Now one thing to note about DHCP

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is that we're going to be using things such as leases we're not just going to give someone an

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IP address indefinitely i.e. they get some type of IP address and that means they will have it

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forever instead we can specify a time frame so maybe we say to someone once they request an IP

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address we say you know you can have this IP address information for one week and after that

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you're going to have to renew that request because simply put think about if we just gave everyone

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who requested an IP address an IP address that they could keep forever and that computer went

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offline ultimately we would just have a lot of dormant and wasted IP address space so really we

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want to be utilizing things such as leases to make sure that the IP address information is only

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granted over a very specific time frame when it has to be renewed. Now it's important to note

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that with DHCP we have the ability to not just grant random IP addresses from a particular

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specified range we can actually have static IP addressing as well meaning that we can actually

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bind an IP address request to a particular MAC address meaning that if the same computer comes

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online with the same MAC address we can always just assign that computer the same address over and over

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again this can be very helpful if we're dealing with things such as servers or printers whereby

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we want to have a very consistent IP addressing scheme so that people can always find the same

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server or the same printer using the same logical IP address as opposed to the server having to call

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it every week or every two weeks or every two days and be assigned a new IP address understand

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with DHCP we still can have that option so now we understand that with DHCP we have the concept of a

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DHCP server as well as a DHCP client the first thing that I now want to dive into is the configuration

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of that DHCP server itself the one that has those DHCP pools the ones that's going to be

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assigning those IP addresses so how on earth do we actually configure DHCP for servers

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well the good news is is that that is what we're going to be talking about in the very next nugget

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so I hope this has been informative for you and I'd like to thank you for viewing