Test Yourself 관련

How IPv4 Works – A Handbook for Developers

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OraThe Internet Protocol version 4 (IPv4) is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 is still the most widely deployed Internet protocol. Google’s IPv6 Statistics show...

• Converting Between Prefix Notation and Subnet Masks
• Working Backwards with Subnet Masks
• Non-Byte-Aligned Prefixes
• Determining Network Membership

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How IPv4 Works – A Handbook for Developers

OraThe Internet Protocol version 4 (IPv4) is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 is still the most widely deployed Internet protocol. Google’s IPv6 Statistics show...

Now practice the concepts you've learned and make sure you feel comfortable with them.

Take a moment to try answering the following questions before checking the answers.

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Converting Between Prefix Notation and Subnet Masks

How would you represent a network prefix of 16 bits, written like this /16, as a subnet mask?

You need 16 bits that are on. When 8 bits are on you get 255 in decimal, so you'd use:

255.255.0.0

16-bit subnet mask address
(Source: BriefVid)

Given this network prefix, do these addresses belong to the same network?

Do these addresses fit in the network defined before?
(Source: BriefVid)

Yes, they do, as they share the same most-significant 16 bits, or two bytes

These addresses fit in the same network
(Source: BriefVid)

Does this address belong to the same network as that of the previous addresses?

Additional address
(Source: BriefVid)

Yes, it does. Again, it shares the same two most-significant bytes.

This address also fits in the network defined before
(Source: BriefVid)

What about this one? Does it belong to the same network as the previous addresses?

Additional address. Does this address fit in the network defined before?
(Source: BriefVid)

No, as the first two bytes are not 42.31 – this is a different network. So this address describes host 1.2, within the network 42.32.

No, this address does not belong to the same network as the other ones
(Source: BriefVid)

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Working Backwards with Subnet Masks

Let's try the other way around. You have this subnet mask:

255.255.255.0

How would you express it using a network prefix?

You have three occurrences of 255, which means three times 8 bits that are on, so overall you have 24 bits that are on. So you can also write /24. This means 3 bytes.

24-bit subnet mask
(Source: BriefVid)

Given this subnet mask, do addresses (1) and (3) above belong to the same network?

Do these addresses have the same network ID given a 24-bit subnet mask?
(Source: BriefVid)

They do, as they both have the same most-significant three bytes – network 42.31.93.

24-bit subnet mask
(Source: BriefVid)

What about addresses (1) and (2)?

Do these addresses have the same network ID given a 24-bit subnet mask?
(Source: BriefVid)

Given this network prefix, they don't belong to the same network. The first address belongs to network 42.31.93, and the second address belongs to network 42.31.1.

24-bit subnet mask
(Source: BriefVid)

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Non-Byte-Aligned Prefixes

Network prefixes do not have to align to 8 bits, or full bytes. Let's say you have a network prefix of 14 bits. How would you convert that to a subnet mask?

Well, the first byte is clear: you have 8 bits on, so the first byte is 255. What about the next one?

In binary, you'd want to have six additional 1s, and then 2 0s – so in binary you'd write:

11111100

Converting to decimal, this binary number represents 252. So your subnet mask is:

255.252.0.0

Another way to make this conversion: You know that eight 1s in binary represent 255 in decimal. You also know that 11 in binary is 3, so you can simply subtract 3 from 255 and get 252.

14-bit subnet mask
(Source: BriefVid)

Next, try the other way around. You have the following subnet mask:

255.255.224.0

How many bits represent the network prefix?

The first two bytes are clear: you have 16 bits. Converting the third byte to binary: 224 in decimal is 11100000 in binary. This means you have an additional three 1s, so you can write the subnet mask above as a prefix of /19 bits – 16 bits for the two 255 bytes, and 3 additional bits for the 224 byte.

19-bit subnet mask
(Source: BriefVid)

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Determining Network Membership

Let's consider the following addresses:

Two IP addresses
(Source: BriefVid)

Are they part of the same network? 🤔

It depends on the subnet mask.

If the network prefix is /8, then they are part of the same network, as they share the same network ID.

8-bit subnet mask
(Source: BriefVid)

On the other hand, if the network prefix is /16, then they have different network IDs, and thus don't belong to the same network. But what happens with prefixes in between? Will they reside in the same network for a prefix of /9? /14?

The way to approach this question is to convert the second byte of these addresses to binary. For the first address, this byte is 24, which in binary is:

00011000

For the second address, the second byte is 23, which in binary is:

00010111

12-bit subnet mask
(Source: BriefVid)

You can see that the most significant 4 bits within the second byte are identical. If you add the first 8 bits of the address, you see that the most significant 12 bits of these addresses are the same.

So, if you have a network prefix of /11, do these addresses belong to the same network?

Yes, they do – their most significant 11 bits are identical.

What about /13?

No, with this network prefix, they don't share the same network identifier, as their 13th bit is different.

This practice should help you feel comfortable with subnet masks and network prefixes. In the next section, you'll learn about special IP addresses and then examine the header of IP packets.