Assigning IP Addresses - CompTIA A+ 220-1101 - 2.5

Summary

This video script explains the evolution of IP address configuration, from manual settings to the introduction of DHCP in 1997. It details the DHCP process, known as DORA (Discover, Offer, Request, Acknowledge), which automates IP address assignment. The script also discusses DHCP server configurations, IP address reservations for consistent device addressing, and APIPA, which assigns local IP addresses when no DHCP server is available.

Takeaways

• 📝 IP addresses, subnet masks, default gateways, and DNS settings are essential for device connectivity in a network.
• 🚀 Manual IP configuration is impractical for large enterprise networks with numerous devices.
• 🔄 BOOTP was the initial protocol for automatic IP assignment but had limitations in configuration management and device tracking.
• 🌟 DHCP was introduced to overcome BOOTP's shortcomings, providing a more comprehensive automatic configuration solution since 1997.
• 🔍 DHCP operates through a four-step process known as D-O-R-A (Discover, Offer, Request, Acknowledge) to assign IP configurations.
• 🔎 The Discover phase involves a broadcast from the client seeking a DHCP server within the local network.
• 🎯 In the Offer phase, the DHCP server responds with an IP address offer to the client, which is sent as a broadcast.
• 📲 The Request phase is where the client selects an offer and sends a broadcast to request the specific IP address from the DHCP server.
• 🗝️ The Acknowledge phase confirms the IP address assignment and provides the client with all necessary configuration settings.
• 🛡️ DHCP servers are configured with a pool of IP addresses to distribute among clients, allowing for dynamic allocation.
• 🔒 DHCP reservations can be set for devices that require a consistent IP address, ensuring they always receive the same configuration.
• 🌐 In the absence of a DHCP server, devices may be assigned an APIPA address, allowing local network communication only.

Q & A

What is the main purpose of DHCP?

The main purpose of DHCP (Dynamic Host Configuration Protocol) is to automatically assign IP addresses and other network configuration settings to devices on a network, eliminating the need for manual configuration on a large scale.

What is the difference between BOOTP and DHCP?

BOOTP (Bootstrap Protocol) was the initial protocol for automatic IP address assignment but had limitations; it did not assign all necessary configuration settings and was unaware of when a device left the network. DHCP was created to address these shortcomings, providing a more comprehensive and dynamic configuration service.

What are the four steps in the DHCP process referred to as D-O-R-A?

The four steps in the DHCP process are: Discover (D), where the device locates a DHCP server; Offer (O), where the server offers an IP address to the device; Request (R), where the device requests the offered IP address; and Acknowledge (A), where the server confirms the IP address assignment.

Why are broadcasts used extensively in the DHCP process?

Broadcasts are used extensively in the DHCP process because the client device does not have an IP address at the beginning, and thus cannot use unicast communication. Broadcasts ensure that messages are sent to all devices on the local network, including the DHCP server.

What is a DHCP proxy and how does it function in the network?

A DHCP proxy, also known as a DHCP relay or IP helper, is a device, such as a router, configured to forward DHCP broadcast messages to a DHCP server on another network segment. This allows devices on different subnets to receive DHCP offers even if they cannot directly communicate with the DHCP server.

What is an APIPA address and why is it used?

An APIPA address (Automatic Private IP Addressing) is used when a device configured to obtain an IP address via DHCP does not detect a DHCP server on the local network. It assigns itself a random IP address within the APIPA range (169.254.0.0 to 169.254.255.255), allowing local network communication but not external connectivity.

How does a DHCP server ensure that certain devices always receive the same IP address?

A DHCP server can be configured with a reservation for specific devices, associating their MAC addresses with a reserved IP address. This ensures that these devices always receive the same IP address upon startup, regardless of network changes.

What is the range of IP addresses used for APIPA?

The range of IP addresses used for APIPA is from 169.254.0.0 to 169.254.255.255. However, the first and last 256 addresses within this range are reserved, so APIPA addresses are randomly assigned between 169.254.1.0 and 169.254.254.255.

What happens if a device receives no response to its ARP request after assigning itself an APIPA address?

If a device does not receive a response to its ARP request after assigning itself an APIPA address, it assumes that the chosen IP address is not in use and proceeds to use it for local network communication.

How can a user check if their device has been assigned an APIPA address?

A user can check if their device has been assigned an APIPA address by running 'ipconfig /all' on a Windows device. The IPv4 address configuration will state that it is an autoconfiguration address, and the IP address will fall within the APIPA range.

Why might a DHCP server not have a default gateway configured for an APIPA address?

A DHCP server does not configure a default gateway for an APIPA address because APIPA addresses are intended for local network communication only. Since these addresses do not route outside the local network, there is no need for a default gateway.

Outlines

00:00

🔄 DHCP and the Process of IP Address Assignment

This paragraph introduces the evolution from manual IP configuration to the Dynamic Host Configuration Protocol (DHCP), which automates the assignment of IP addresses and other network settings. It explains the limitations of the Bootstrap Protocol (BOOTP) and how DHCP, introduced in 1997, improved upon it by providing a full range of configuration settings. The paragraph outlines the four-step process known as D-O-R-A (Discover, Offer, Request, Acknowledge) that DHCP uses to assign an IP address to a device. It also includes a brief description of a network diagram involving a client workstation, a local DHCP server, and a router configured as a DHCP proxy, illustrating how the process works in a practical scenario.

05:00

📁 Understanding DHCP Reservations and APIPA

The second paragraph delves into the concept of DHCP reservations, which allow specific devices like printers or servers to always receive the same IP address, ensuring consistency and reducing administrative overhead. It contrasts this with the manual configuration of IP addresses and the challenges it presents in large networks. The paragraph also introduces APIPA (Automatic Private IP Addressing), a mechanism that assigns an IP address to a device from a predefined range when no DHCP server is available. This results in a link-local address that restricts communication to the local network only. The standard range for APIPA addresses and the process by which a device selects an APIPA address are explained, along with an example of how an APIPA address appears on a Windows device using the 'ipconfig/all' command.

Keywords

💡IP Address

An IP address is a numerical label assigned to devices participating in a computer network that uses the Internet Protocol for communication, allowing them to be identified and located within the network. In the video, the manual configuration of an IP address is discussed, which includes setting up the address, subnet mask, default gateway, and DNS settings. The script also explains the importance of IP addresses in the context of DHCP, where devices are automatically assigned these addresses to ensure scalability in large networks.

💡Subnet Mask

A subnet mask is used in networking to divide a network into smaller subnetworks, or subnets, allowing for more efficient allocation of IP addresses and better organization of networks. In the video, it is mentioned as part of the manual IP configuration process and is essential for defining the network boundary and determining which devices are in the same local network.

💡Default Gateway

The default gateway is a network node on the path used by routers to forward packets outside of the local network. It is a crucial part of IP configuration, as it directs traffic to and from external networks. In the context of the video, the default gateway is part of the settings that can be manually configured or automatically assigned by DHCP.

💡DNS Setting

DNS, or Domain Name System, settings are configurations that specify the IP addresses of DNS servers used to resolve domain names into IP addresses, enabling users to access websites using human-readable domain names instead of numerical IP addresses. The video script explains that these settings are part of the manual IP configuration and are also automatically assigned by DHCP.

💡BOOTP

BOOTP, or Bootstrap Protocol, was the precursor to DHCP and was used to automatically assign IP addresses to devices. However, as mentioned in the video, it had limitations in that it did not assign all necessary configuration settings and was unaware of when a device left the network. BOOTP laid the groundwork for the more advanced DHCP.

💡DHCP

DHCP, or Dynamic Host Configuration Protocol, is a network management protocol used to dynamically assign IP addresses and other network configuration parameters to devices on a network. The video explains that DHCP was created to address the shortcomings of BOOTP and has been in use since 1997, providing an automatic configuration process for various network settings.

💡DORA

DORA is an acronym representing the four-step process that DHCP follows to assign IP configurations to devices: Discover, Offer, Request, and Acknowledge. The video script details each step, illustrating how a device locates a DHCP server, receives offers for IP addresses, makes a request for one of those addresses, and finally receives an acknowledgment with the configuration settings.

💡DHCP Server

A DHCP server is a server that manages IP address assignment to devices on a network using the DHCP protocol. The video script describes the role of the DHCP server in the DORA process, offering IP addresses to devices, and sending acknowledgments with the necessary network configuration settings.

💡DHCP Proxy

A DHCP proxy, also known as a DHCP relay or IP helper, is a device that forwards DHCP broadcast messages from a client on one network to a DHCP server on another network. The video script explains how a router can be configured as a DHCP proxy to allow broadcasts to pass through and reach a DHCP server on a different subnet.

💡APIPA

APIPA, or Automatic Private IP Addressing, is a method by which devices can automatically self-assign an IP address when a DHCP server is not available. The video script describes the APIPA process, including the range of IP addresses used (169.254.0.0 through 169.254.255.255) and how devices use ARP to ensure the selected IP address is not already in use on the local network.

💡IP Reservation

IP reservation is a feature in DHCP that allows network administrators to reserve a specific IP address for a device, ensuring it always receives the same address upon network connection. The video script discusses the use of IP reservations for devices like printers or servers, which should consistently have the same IP address for ease of access and management.

Highlights

Manual IP configuration includes an IP address, subnet mask, default gateway, and DNS setting.

Manual IP configuration does not scale well in large enterprise networks.

BOOTP was created in October 1993 to automatically assign IP addresses to devices.

BOOTP had limitations in assigning all necessary configuration settings and managing device departures from the network.

DHCP was introduced as an improved version of BOOTP in 1997, providing automatic configuration for more settings.

The DHCP process involves four steps: Discover, Offer, Request, and Acknowledge (DORA).

DHCP servers can be located using broadcasts sent by devices to UDP port 67 during the Discover phase.

Routers configured as DHCP proxies can forward DHCP broadcast requests to other networks.

DHCP servers respond to client requests with offers sent to UDP port 68 during the Offer phase.

Clients select an offer and send a broadcast request to the DHCP server on UDP port 67 in the Request phase.

DHCP servers acknowledge the request and provide IP configuration settings during the Acknowledge phase, using UDP port 68.

DHCP servers manage a pool of IP addresses to assign to clients, which may vary with each connection.

Reservations on DHCP servers can ensure certain devices always receive the same IP address.

Disabling DHCP and manual IP configuration on devices is an alternative but requires administrative changes.

APIPA assigns an Automatic Private IP Address when no DHCP server is available on the local network.

APIPA addresses are limited to local network communication and are randomly assigned within a specific range.

APIPA uses ARP to check for IP address conflicts before assigning an address to a device.

Windows devices with APIPA addresses show 'autoconfiguration' in the IPv4 address configuration.

Transcripts

In a previous video, we talked about a manual configuration

of an IP address that includes an IP address, a subnet mask,

a default gateway, and a DNS setting.

And of course, there are other IP configuration settings

as well.

You can imagine on an enterprise network, where

you have hundreds or even thousands of devices,

manually configuring IP addresses is not

going to scale very well.

And that's why in October of 1993,

we created a protocol that can automatically

assign IP addresses to devices.

We refer to this as the bootstrap protocol or BOOTP.

BOOTP was a good start, but it didn't

assign all of the different configuration settings

that we needed.

There were still some manual configurations

that you had to do yourself.

And BOOTP had no idea when a device would leave the network,

so that it could use that IP address for another device

later on.

To correct some of these shortcomings,

we created a new version of BOOTP.

And we gave it a new name, the Dynamic Host Configuration

Protocol or DHCP.

DHCP has been around for quite some time since 1997.

And it provides an automatic configuration

of IP addresses, subnet masks, default gateways, and many more

settings as well.

We often don't even think about the process

that DHCP goes through to assign these values to your computer.

We simply turn on our system, and it magically

has an IP address once it boots up.

But behind the scenes there are a series

of steps that take place to provide that IP

configuration to your device.

We refer to these four steps as D-O-R-A or Dora.

The first of these is the D step or discover,

where our devices will try to locate a DHCP server that's

on our local network.

The next step of O is to offer where a DHCP server will offer

an IP address to our device.

Our device will then look at the one or many offers

that we receive, pick one of those offers,

and then request that IP address from the offering DHCP server.

And finally, the DHCP server will acknowledge that offer

and provide your system with all of the IP configuration

settings that it needs.

Let's look at each of these steps

to see exactly how this process occurs on our network.

Here's a simple network diagram that

includes my client workstation, which is connected to a switch.

And that switch I also have connected a local DHCP

server on my local subnet.

And I have a router that's on my network as well.

On the other side of that router,

on a separate IP subnet, is another DHCP server

so that we can have some type of redundancy

if either one of those was to fail.

The step one of the DHCP process or the discover part

of the process has my client workstation

sending a broadcast out to the network to UDP port 67.

This broadcast will be sent to all devices

on my local network.

And that broadcast is going to be

sent both to the router that's on my network and the DHCP

server that's on my network.

One of the things that you probably

remember from looking at broadcast multicast and unicast

is that routers will not allow broadcast to pass.

But you'll notice in our example,

the broadcast did pass through this router

and was directed to a DHCP server.

That's because we've effectively configured this router

as a DHCP proxy.

You'll sometimes see this referenced as a DHCP relay,

and some manufacturers call it an IP helper.

This obviously requires that you pre-configure this router

so that any broadcasts associated with DHCP

are automatically forwarded to a DHCP server

elsewhere on the network.

Now that these two DHCP servers have received our request,

we can move to step two or the offer phase.

These two devices will send their offers back

to the original client workstation.

This is often sent to a broadcast address over UDP 68.

The reason we're using so many broadcasts

is because, remember, our client workstation does not yet

have an IP address.

Now that this message is sent to the client workstation,

it can examine both of those offers

and make a decision on which offer

it should take for the IP address assignment.

The client workstation will send another broadcast

over UDP port 67 on step three or the request phase.

This broadcast contains a formal request

to take the offer that was originally sent.

And in this particular example, I'm

only showing the communication that's

occurring between the client workstation and the DHCP server

that made the original offer.

But of course, since this is a broadcast,

it would be sent to all other devices on this local subnet.

And now that the DHCP server has received this formal request,

it can begin step four or the acknowledgment phase

where it sends a message back to the client workstation

by default over UDP port 68 confirming

that the request from the previous phase

has been acknowledged, and this device can assign itself

with the configuration settings included in the offer.

This four step DORA process happens every time

a device connects to the network and needs to obtain an IP

address from a DHCP server.

When these DHCP servers were first configured,

a range of IP addresses was configured inside

of that server to hand out to the different clients.

We refer to this as a pool of IP addresses.

And the IP address that you may pull one day from this pool

may be different than when you might use on a different day.

But there might be some devices on your network

that you would prefer always receive

exactly the same IP address every single time it starts up.

There might be a printer, a server,

or some type of infrastructure device

that you would prefer to always have the same IP.

One of the ways you could do this

is to simply disable DHCP on all of these devices

and manually configure the IP addresses yourself.

This obviously requires that you administratively

make this change.

And if you make any changes to the IP address settings

on your network, then you'll manually

need to revisit these devices to change

all of your previous configurations.

In a very large environment this may not be very practical.

And it certainly requires additional administration.

So instead of manually setting an IP address,

we can configure a reservation on the DHCP server.

This means we'll configure a specific MAC address

of a printer, a server, or another device on the network,

and tell our DHCP server every time you see this MAC address,

always provide it with this reserved IP address.

This way every time we start up the printer or the server,

it will always have exactly the same IP address.

This is the best practice for IP address configurations

because we know it's so easy to set a manual address.

The problem is that when there are changes to the network,

you'll have to go back to every single one of those devices.

So having a reservation already in the DHCP server

can make this process so much easier

than revisiting all of these different devices

on your network.

There may be times when your device is configured

to obtain a DHCP address, but on your local network

there's no longer a DHCP server.

In those situations your device will

be assigned an APIPA address.

That stands for an Automatic Private IP Addressing Address.

This is sometimes referred to as a link-local address,

because any device that's configured with an APIPA

address range can only communicate

to other devices on the local network.

If your device is assigned an APIPA address,

it can't communicate outside of your local network.

Like most things, there is a standard

for the automatic private IP addressing.

The entire range is 169.254.0.0 through 169.254.255.255.

This particular standard reserves the first and last 256

addresses.

So if you are assigned an APIPA address,

it will be a random assignment between 169.254.1.

0 through 169.254.254.255.

If you see any of those IP addresses on a system,

then you can automatically assume

that a DHCP server was not available

when the system started up.

And you will only be able to communicate with other devices

on your local network.

Like DHCP, APIPA addresses are also assigned to automatically.

But the only device determining what IP address you receive

is your local device itself.

Your local device will pick a random IP address that's

in this APIPA address range.

It will perform an address resolution protocol or ARP

request to see if anybody else on the local network

is currently using that IP address.

And if it doesn't receive a response,

it assigns signs that APIPA address to your local device.

Here's what you would see if you ran

an ipconfig/all on a Windows device

that has an APIPA address assigned.

You can see that the IPv4 address configuration even

states that it is an autoconfiguration address.

And it is assigned 169.254.228.109

with the default subnet mask for APIPA of 255.255.0.0.

And because you can't communicate outside

of this local network, you can see that the default gateway

option has been left blank.