TCP/IP Addressing – CompTIA A+ 220-901 – 2.3

Our computers must be configured with the appropriate IP addressing to properly communicate across the network. In this video, you’ll learn about IP addressing, an overview of DNS, DHCP configurations, and the basics of classless subnetting.

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When you’re configuring a TCP IP address on a workstation, you generally need three things. And the first one, of course, is going to be the IP address itself. Usually this is an IPv4 address, so it will have four numbers associated with it, like Every device on your network is going to have a completely different and unique IP address.

Along with this IP address, you must also have a subnet mask. The subnet mask is going to have the same four numbers. In this case, it might be This is a mask that lets this local workstation know what network it belongs to. This is not something that is transmitted across the network. Your IP address is what’s sent across the network. The subnet mask is used by the local device.

So you’re always going to get these two things together. If you don’t get a subnet mask with your IP address, you’re going to have to find out what it is. You may have to ask your network administrator, what is the subnet mask that I’m going to use with this IP address?

If you’re going to be communicating to devices on the internet or even just outside of your local network, you’re also going to need a default gateway. This is going to be the IP address associated with the router that’s on your local network you will not be able to communicate outside of your local network unless you have assigned a default gateway.

An IP configuration option that is optional but almost always used is one for a domain name system IP address. This is used to convert the names you provide to your computer into an IP address, so that that device can then be communicated with across the network. You only have to remember, but behind the scenes, your computer goes to a DNS server, finds out what the IP address is of that particular device, and uses that to communicate back to the ProfessorMesser web server.

DNS is a distributed database, with servers all over the world. Everything starts with these root server clusters. There’s 13 root servers for DNS, and from there it breaks off into many other servers. There are hundreds of top-level domains. We’re familiar with .com and .org and .net, but there are also top-level domains for countries. These country code top-level domains are for things like .us or .ca for Canada, or .uk for the United Kingdom.

In your environment, your network administrator has created and probably manages multiple DNS servers inside of your environment, and you’re usually given two of those addresses, just to make sure there is redundancy. That way, if you can’t communicate to one DNS server, there will generally be another one that you can fall back to.

In the early days of IP networking, all of these configuration settings had to be configured manually. We call this static addressing, where you go into the configuration of your computer and you add the IP address and the subnet mask and the default gateway and the DNS server IP addresses, and anything else that’s required for IP communication.

Of course, if you’re putting in all of these configuration settings by hand, you’re bound to make a mistake. And if you have a lot of different computers, there’s a lot of management overhead, because you have to go to every single computer to configure the IP address settings. That’s why we came up with DHCP.

DHCP is the Dynamic Host Configuration Protocol, and it is the protocol that automatically assigns all of the settings. All you have to do is plug in your computer to the network, and it magically figures out what all of these different configuration settings are. In reality there’s no magic at all. It’s simply using the Dynamic Host Configuration Protocol to assign all of this inside of your computer.

So you’re usually presented with the option to either use a statically assigned address or obtain an IP address automatically, and that’s using DHCP. You might also have the same option for your DNS settings, so you could configure DNS statically, or again, go to your DHCP server to get all of your DNS configuration settings. You also have the ability to turn these things on or turn these things off as you wish, but generally, you keep everything to obtain an IP address automatically, and your computer will find the closest DHCP server and assign all of that for you without any user intervention.

In a previous video, we described a subnet mask, not in its decimal form like, but we expressed it as a CIDR block notation or /24. CIDR stands for Classless Inter-Domain Routing. This notation type removed a lot of the earlier restrictions we have with class-based addressing, and that’s why we call this the Classless Inter-Domain Routing.

This flexibility with the subnet mask allowed us to get rid of a lot of restrictions associated with earlier forms of addressing, and that’s why we have this Classless Inter-Domain Routing of CIDR, which we often call “cider” block notation. You might see subnet mask referenced in decimal notation, or you might see them referenced in CIDR block notation. It’s just two ways to write exactly the same thing.

This is really describing how many bits are going to be used in the subnet mask. For example, the subnet mask of can be written out in binary form, with all ones in the first octet and all zeroes in the remaining octet. So all ones in that first octet is the 255, and then of course you have the all zeroes. Because there are eight ones in this subnet mask, we use a CIDR block notation of /8.

We do the same thing with a subnet mask of So if you wrote this out in decimal form, there would be 16 ones in that subnet mask, and that means the CIDR block notation is /16. So it really doesn’t matter what form it’s taking. It can be decimal or it can be CIDR block notation, but ultimately, it’s referring back to the subnet mask on that device.

Whether you use a decimal subnet mask or CIDR block notation as the subnet mask is generally going to depend on the operating system or the application that you’re using. You’ll almost always have an IP address that needs to be assigned with the subnet mask. You’ll almost always provide a default gateway. And if you’re going to be communicating on the Internet, there is always going to be a DNS server associated with this.

Since some operating systems are expecting a decimal subnet mask and other operating systems are expecting a CIDR block notation, you want to memorize exactly the format, so that you can convert back and forth between them.