How to subnet in third octet IPv4

  • Thread starter itlivesthere
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This gives you the first usable address.The last usable address is found by setting all bits from bit 20 up to bit 32 to one.The broadcast address is found by setting all bits from bit 20 up to bit 32 to one.In summary, the conversation discusses the difficulties of subnetting Class B addresses and the confusion around spacing out the address blocks in the third octet when dealing with networks below /24. The speaker also mentions the need for easy-to-memorize tricks for subnetting Class B and A addresses with paper and pencil. They then provide a breakdown of how to find the network number, first and last usable host, and broadcast addresses for an example with an IP address of 150.53.
  • #1
itlivesthere
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I already know how to subnet Class C addresses. Class B subnetting isn't proving to be very difficult, except for one little thing. I'm confused about how to space out the address blocks in the third octet when dealing with networks below /24.

Here's an example that I'm having trouble with:

Given the IP address 150.53.218.209/19 provide:
Network Number:
First Usable Host:
Last Usable Host:
Broadcast:
The reason this confuses me is that the third and fourth octets are already mostly made of 1's (binary), therefore doesn't that mean not many addresses are available right off the rip?
Also, if anyone has some easy-to-memorize tricks to subnet Class B and A addresses that I can do with paper and pencil I would greatly appreciate the help!
 
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  • #2
Welcome to PF, itlivesthere! :smile:

The first 19 bits identify the network (subnet), the last 13 bits identify the client.
To find the network number, you need to set all bits from bit 20 up to bit 32 to zero.
 

1. What is a third octet in IPv4?

The third octet in IPv4 refers to the third set of numbers separated by periods in the IP address. It is used to identify a specific network within a larger network.

2. How do I subnet in the third octet of an IPv4 address?

To subnet in the third octet of an IPv4 address, you will need to use the subnetting technique known as "Classless Inter-Domain Routing" (CIDR). This involves borrowing bits from the third octet to create smaller subnetworks.

3. Why is subnetting in the third octet useful?

Subnetting in the third octet allows for more efficient use of IP addresses by creating smaller subnetworks within a larger network. This can help with network management and organization.

4. What is the maximum number of subnets that can be created in the third octet of an IPv4 address?

The maximum number of subnets that can be created in the third octet of an IPv4 address depends on the number of bits that are borrowed from the third octet. For example, if 2 bits are borrowed, there can be a maximum of 4 subnets.

5. How do I determine the subnet mask for a third octet subnetted IPv4 address?

To determine the subnet mask for a third octet subnetted IPv4 address, you will need to know the number of bits that were borrowed and add that to the default subnet mask for a Class C network (255.255.255.0). For example, if 2 bits were borrowed, the subnet mask would be 255.255.255.192.

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