Free CCNA 200-301 Course 06-07: The Subnet Mask

Summary

This video delves into the concept of subnet masks, a crucial aspect of IP networking. It explains how subnet masks determine the boundary between the network and host portions of an IP address, enabling devices to identify whether a destination is on the same subnet or a different one. Through clear examples and binary representation, it illustrates how subnet masks work and the significance of contiguous blocks of ones and zeros. Additionally, it covers rules for assigning host addresses, highlighting the reserved network address and broadcast address. Overall, this video provides a comprehensive understanding of subnet masks and their role in IP addressing and routing.

Takeaways

• 🔗 The video provides a link to download hands-on lab exercises that accompany the CCNA course.
• 🌐 Hosts on the same subnet can communicate directly with each other, but communication between subnets requires a router to forward the traffic.
• 🎭 The subnet mask helps hosts determine if the destination IP address is on the same subnet or a different one.
• 🔢 The subnet mask is a 32-bit binary number, represented in dot-decimal or slash notation, that separates the network and host portions of an IP address.
• 🔍 The host compares its IP address and subnet mask in binary to identify the network and host portions of the address.
• 🚦 The network portion of the IP address must match for hosts to be on the same subnet and communicate directly.
• 🚫 Two special addresses cannot be assigned to hosts: the network address (all zeros in the host portion) and the directed broadcast address (all ones in the host portion).
• 📝 The host portion of the IP address is used to assign unique addresses to devices on the same subnet.
• 🔀 Host IP addresses do not need to be sequential within a subnet, but they cannot be duplicates.
• 📚 The video promotes the instructor's CCNA Gold Bootcamp course, which includes additional study materials and troubleshooting labs.

Q & A

What is a subnet mask and how is it represented?

A subnet mask is a 32-bit number that divides an IP address into network and host portions. It can be represented in dot-decimal notation (e.g., 255.255.255.0) or in slash notation (e.g., /24).

How does a host determine if a destination IP address is on the same subnet or a different subnet?

A host compares the network portion of the destination IP address with its own network portion, which is determined by applying the subnet mask to its own IP address. If the network portions match, the destination is on the same subnet; if they differ, the destination is on a different subnet.

What is the purpose of a router in a network with subnets?

A router is a device that links different subnets together and routes traffic between them. If a host needs to send traffic to a destination on a different subnet, it must be forwarded by a router.

Can the ones and zeros in a subnet mask be mixed in any order?

No, the subnet mask must have a contiguous block of ones followed by a contiguous block of zeros. Mixing the ones and zeros in a subnet mask is not valid.

What is the significance of an all-zeros host portion in an IP address?

An all-zeros host portion designates the network address and cannot be assigned to a host. It is the lowest address in the subnet range.

What is the significance of an all-ones host portion in an IP address?

An all-ones host portion represents the directed broadcast address, which sends traffic to all hosts on that subnet. It cannot be assigned to an individual host and is the highest address in the subnet range.

Can two hosts on the same subnet have the same IP address?

No, two hosts on the same subnet cannot have the same IP address. Duplicate IP addresses on the same subnet are not allowed, as network devices would not know which host to send traffic to.

Do hosts on a subnet need to be numbered sequentially?

No, hosts on a subnet do not need to be numbered sequentially. As long as the IP addresses are unique on that subnet, they can be assigned in any order.

Can two hosts on different subnets have the same host portion of their IP addresses?

Yes, two hosts on different subnets can have the same host portion of their IP addresses, as long as the network portions differ.

What is the purpose of the provided hands-on lab exercises mentioned in the script?

The hands-on lab exercises accompany the CCNA course and provide practical experience for learners to reinforce the concepts covered in the lectures, such as subnet masks and IP addressing.

Outlines

00:00

📺 Introduction and IP Configuration

This paragraph serves as an introduction, reminding viewers to download the accompanying lab exercises, subscribe, and enable notifications. It then moves on to demonstrate the IP configuration on the speaker's Windows laptop, highlighting the IP address (192.168.10.15), subnet mask (255.255.255.0), and default gateway (192.168.10.x). The paragraph sets the stage for explaining how hosts use this information to communicate within and between subnets.

05:01

🌐 Subnet Masks and Communication Between Subnets

This paragraph delves into the role of subnet masks in determining whether a host can communicate directly with another host or needs to route traffic through a router (default gateway). It explains how hosts compare the destination IP address with their own IP address and subnet mask to determine if the destination is on the same subnet or a different one. The paragraph also introduces the binary representation of IP addresses and subnet masks, illustrating how the subnet mask separates the network portion from the host portion of an IP address.

10:01

🏠 Host Addressing and Subnet Ranges

This paragraph focuses on the host portion of an IP address and the valid range of addresses that can be assigned to hosts within a subnet. It explains the significance of all zeros (network address) and all ones (directed broadcast address) in the host portion, which cannot be assigned to individual hosts. The paragraph also clarifies that IP addresses do not need to be assigned sequentially within a subnet and that duplicate IP addresses are not allowed on the same subnet. Finally, it provides an example subnet range (192.168.10.1 to 192.168.10.254) that can be assigned to hosts.

Keywords

💡IP Address

An IP address is a unique numerical label assigned to each device connected to a computer network. It allows devices to communicate with each other over the network. In the video, the instructor shows their laptop's IP address (192.168.10.15) and explains how IP addresses help hosts determine whether a destination is on the same subnet or a different subnet, enabling them to send traffic directly or via a router.

💡Subnet Mask

A subnet mask is a 32-bit number that separates an IP address into the network portion and the host portion. It defines the boundary between the network and host addresses. The video uses the subnet mask 255.255.255.0 (expressed in binary notation) to illustrate how the subnet mask determines which part of the IP address belongs to the network and which part belongs to the host.

💡Default Gateway

The default gateway is a router on the same network as the host, which is responsible for forwarding traffic to other networks or subnets. In the video, the instructor mentions that the host needs to send traffic destined for a different subnet to the default gateway (192.168.10.1 in the example), which then routes the traffic to the appropriate destination network.

💡Subnet

A subnet is a logical subdivision of an IP network, created by dividing the network's address space into smaller, more manageable segments. In the video, the instructor explains that hosts on the same subnet can communicate directly, while communication between hosts on different subnets requires a router to forward the traffic.

💡Router

A router is a networking device that connects multiple networks or subnets and forwards data packets between them. The video states that routers are devices that link different subnets together and route traffic between them. Hosts must send traffic destined for a different subnet to their local router (default gateway) for forwarding.

💡Binary Notation

Binary notation is a way of representing numbers using only two digits, 0 and 1. In the video, the instructor demonstrates how IP addresses and subnet masks are converted to binary notation to better understand the network and host portions of the addresses, and how the subnet mask is used to determine the boundary between them.

💡Network Address

The network address, also known as the network ID, is a unique identifier for a specific network or subnet. In the video, the instructor explains that an IP address with all zeros in the host portion (e.g., 192.168.10.0) represents the network address and cannot be assigned to a host.

💡Directed Broadcast Address

The directed broadcast address is a special IP address used to send data to all hosts on a specific subnet. In the video, the instructor mentions that an IP address with all ones in the host portion (e.g., 192.168.10.255) represents the directed broadcast address and cannot be assigned to an individual host because it would send traffic to all hosts on that subnet.

💡Host Address

A host address is a unique identifier assigned to a specific device or host within a network or subnet. In the video, the instructor explains that the host portion of an IP address (e.g., .15 in 192.168.10.15) represents the individual host address, which must be unique on that subnet.

💡Duplicate IP Addresses

Duplicate IP addresses refer to two or more devices on the same network or subnet having the same IP address assigned to them. The video emphasizes that duplicate IP addresses are not allowed, as it would cause confusion for network devices trying to send traffic to that IP address, as they wouldn't know which host to send it to.

Highlights

A host can send traffic directly to another host on the same subnet via the switches that they're attached to, but for a host to send traffic to another host in a different subnet, it must be forwarded by a router.

The host needs to understand if the destination is on the same or a different subnet in order to know how to send it.

If the destination is on the same subnet, it will send it there directly. If it's on a different subnet, it knows that it has to send it to the local router, which is the default gateway.

The way that the host knows whether the destination is on the same subnet or a different subnet is by comparing the IP address of the destination to its own IP address and subnet mask.

The subnet mask defines where the boundary is between the network part and the host part of the address.

A 1 in the subnet mask indicates that bit in the IP address is part of the network address, and a 0 indicates that the bit is part of the host address.

The subnet mask always begins with a contiguous block of ones, followed by a block of zeros. The ones and zeros cannot be mixed up in the subnet mask.

The host portion of the address is available to be allocated to the different hosts on that particular subnet, such as PCs, servers, printers, router interfaces, and switch management addresses.

The host portion of the address specifies the individual host and must be unique on that subnet.

All zeros in the host portion designates the network address and is not allowed to be allocated to a host.

All ones in the host portion of the address is the directed broadcast, which goes to all hosts in that subnet, and cannot be assigned to an individual host.

In the example of 192.168.10.0/24, the range of addresses available to be allocated to hosts is 192.168.10.1 to 192.168.10.254.

Hosts on the same subnet can send traffic directly to each other without going through the default gateway router.

The complete CCNA course, including hands-on lab exercises and additional study materials, is available for enrollment.

The lecture explains how hosts determine whether a destination is on the same subnet or a different subnet, and how they use the subnet mask to identify the network and host portions of an IP address.

Transcripts

just a quick reminder before we get into

the lesson

to download the hands-on lab exercises

that accompany this complete ccna course

i'll include the link in the description

also remember to subscribe

and hit the notifications bell so you

don't miss any of the lessons in the

course

okay let's get into it

in this lecture you're going to learn

about

subnet masks and you can see what i've

done here is i've opened up

a command prompt on my windows laptop

again

and i've entered ipconfig and you can

see where i've highlighted it

that my ip address is 192.168.10.15.

my subnet mask is 255.255.255.0

and the default gateway is 192.168.10.

so every host in your network is going

to know

what its ip address its subnet mask

and its default gateway is

let's now see how it's going to use that

information

so onto the slides now a host can send

traffic directly to another host on the

same subnet

via the switches that they're attached

to

but for a host to send traffic to

another host in a different subnet

it must be forwarded by a router so our

routers are devices

that link our different subnets together

and route the traffic between them

the host therefore needs to understand

if the destination

is on the same or a different subnet in

order to know how to send it

if the destination is on the same subnet

it will send it there

directly if it's on a different subnet

it knows that it has to send it to the

local router

which is the default gateway and the way

that the host knows

whether the destination is on the same

subnet or a different subnet

is by comparing the ip address

of the destination to its

own ip address and subnet mask

the subnet mask just like the ip address

is also 32 bits long and it can be

written

in dot decimal notation the same as our

ip

addresses or it can be written in slash

notation

you'll see how that works a bit later in

this lecture

a host's ip address is divided into a

network portion and a host portion

and it's the subnet mask that defines

where the boundary is between the

network part and the host part of the

address

and the easiest way to explain how this

works is by giving you

an example so let's say the host's ip

address is

192.168.10.50

and the subnet mask is 255.255.255.0

that's the ip address and subnet mask

it's actually on my

laptop to figure this out we write

the ip address out in binary notation

like you learned in the last lecture

and then the subnet mask also in binary

notation

underneath so our example was

192.168.10.15

subnet mask 255.255.255.0

so you see the the top part here i've

written

192.168.10.15 out in binary

and then underneath 255.255.255.0

out in binary as well the

ip address is compared or masked

with the subnet mask a 1 in the subnet

mask

indicates that bit in the ip address is

part of the network address and a zero

indicates that the part that the bit is

part of

the host address so very quickly you can

see here

all the ones on the subnet mask go up to

here

everything in the ip address above that

is part of the network portion of the

address

the zeros above that way in the

ip address those are part of the host

portion of the address let's make this a

little bit clearer

so subnet mask 255.255.255.0

with the subnet mask it's always going

to have contiguous ones and you see the

ones come up to this part here

so i put a line in that line

is the border between the network

portion and the host portion

of the address so in the example the

network address portion is 192.168.10

because on the ip address from here

on the left all the way up to the line

that is 192.168.10

part of the ip address whatever is

after the line is the host portion of

the address so in our example is the dot

15

is the host portion of the address

and i've highlighted it there there is

the network portion

if the host wants to communicate with

another

host with an ip address which also

begins with 192.168.10

in our example so say for example

that this host wants to send traffic to

a destination address of 192.168.10.20

it knows it's on the same subnet and it

can send the traffic directly

because the destination also begins with

192.168.10.

if this host wants to communicate with

another host on

any other network anything that does not

begin with 192.168.10

then it knows it has to send the traffic

via a router

so if it was sending traffic to

destination 192.163

for example it doesn't begin with

192.168.10

it's a different subnet sends it via the

router

for a destination address to be on the

same subnet the network portion has to

be

exactly 192.168.10.

anything else means it's a different

subnet we have to go via a router

the subnet mask always begins

with a contiguous block of ones

this is different than the ip address

you see our example i p address here

it's one one zero zero zero zero zero

zero

one zero one zero one so with the i ip

address the ones and the zeros can be

mixed about in pretty much any order the

subnet mask

is a block of ones and then a block of

zeros

always we never mix the ones and the

zeros up with each other

in the subnet mask so one one one one

one one one dot one one one one

zero zero zero zero dot zero zero zero

etcetera is a legal subnet a mask one

one one zero one one zero

one mixing up the ones and zeros we

can't do that that is not a valid subnet

mask

the host portion of the address is

available to be allocated

to the different hosts on that

particular subnet

for example your pcs servers printers

router interfaces switch management

addresses etc

with two exceptions that you'll see

coming up after the next slide

so there's the host portion of the

address for our example highlighted

the host portion of the address

specifies the individual host

and must be unique on that subnet

your hosts do not need to be numbered

sequentially for example we could have a

subnet with two hosts on it

one could have addressed 10.10.10.10 the

other could have addressed 10.10.10.20.

we don't need to number them dot one and

dot two

you can't have two different hosts both

with the same ip address for example we

couldn't have two hosts both with

address 10.10.10.10

that would be a duplicate address and

whenever

any traffic was sent to 10.10.10.10

your network devices wouldn't know which

host to send it to so if that's illegal

you can't have duplicate ip addresses

you could have host 10.10.10.

on one subnet and host 10.10.20.10 on a

different subnet

they're different subnets so it's not a

duplicate address that's just fine

all zeroes in the host portion

designates the network address and is

not allowed to be allocated to a host

remember we just said

a minute ago that there's two particular

addresses that cannot be assigned to a

host

the first one of those is all zeroes in

the host portion

that designates the network address or

the network id

in our example the network address would

be 192.168.10.0

so we fill in the bit pattern in the

network portion so that was 192.168.10

and then in the host portion we put all

zeros in there so all zeros you can't

assign it to

a host it signifies the

network address which is the the bottom

address in that particular subnet

and there's highlighted there you can

see we've used all zeros

the other address which cannot be

assigned to a host is

all ones in the host portion of the

address so all

zeroes signifies the network address

which is the bottom address in the range

all ones which is the top address in the

range

that is the directed broadcast

whenever you send traffic to the

directed broadcast address

it goes to all hosts in that subnet

not to an individual host so we can't

assign that address to an individual

host

and there it is highlighted the host

portion i've put all ones in there

so that leaves 192.168.10.1

to 192.168.10.254.

in our example available to be allocated

to our different hosts so all the

different pcs

other kinds of hosts maybe we've got

some windows pcs some linux pcs

in that subnet i can number them from

192.168.10.1

all the way up to winding 2.168.10.254.

they're all in the same subnet whenever

they send traffic to each other they can

do that directly

without going via their default gateway

router

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