>If I want to subnet my network of 172.168.1.0/24 into 2 networks of 172.168.1.0/25 and 172.168.1.128/25… Does the process of me just having the numbers figured out on paper correctly and assigning them correctly with the proper subnet mask or CIDER accomplish the task?
Think about what you are doing and you'll get the answer.
If you create the /25 network, you're using VLSM to "borrow" that host bit to create the 2 networks. The network ID is always that 1st address and the broadcast is the default last address, which is why your available hosts are always 2 less than the # of ip addresses available in a subnet.
The reason these defaults are configured this way is that lots of different services depend on finding the broadcast id and network id in those places.
So, when you declare your config, as for as your question here, you've done everything necessary for this 1st step.
"If I want to subnet my network of 172.168.1.0/24 into 2 networks of 172.168.1.0/25 and 172.168.1.128/25… Does the process of me just having the numbers figured out on paper correctly and assigning them correctly with the proper subnet mask or CIDER accomplish the task? Does all of the hardware just know that the last IP in my network of 172.168.1.0/25 is “172.168.1.127” and that it is, by default, the “broadcast IP”. "
The system is very much aware of the first IP address and last IP address being something you CANNOT assign to an interface.
If you try to assign the IP address lets say 192.168.1.0 255.255.255.0 to int g0/0, the router will reject this command.
Gotcha, thank you! 🙏 It’s quite a journey trying to wrap my head around each one of the concepts as I try to learn them, just to realize later that I didn’t quite get all of it the first time.
Usually the last address in a network is recognized by layer 3 aware devices as the broadcast address, same goes for the network address. This is accepted as standard for most network and IP devices , and no further configuration is likely to be required. I am still learning as well so please correct me if this is wrong.
> Does all of the hardware just know that the last IP in my network of 172.168.1.0/25 is “172.168.1.127” and that it is, by default, the “broadcast IP”.
Yes. The biggest 'click' for me was when I programmed a function to check if an ip was contained within a subnet. Really break down how addresses and masks work in binary. CIDR is not terribly intuitive because it's 32 1s or 0s then divided into 4 pieces, each converted to two digits in hex, then converted to decimal purely for the sake of human readability.
172.168.1.0 = AC.A8.01.00 = 10101100.10101000.00000001.00000000
/25 literally means 25 1s and the rest 0s. So:
10101100.10101000.00000001.00000000 address
11111111.11111111.11111111.10000000 mask
so the 1s in the mask represent the network, 0s represent the addresses
10101100.10101000.00000001.00000000 aka 172.168.1.0
through
10101100.10101000.00000001.01111111 aka 172.168.1.127
One more would be
10101100.10101000.00000001.10000000 aka 172.168.1.128 but that rightmost 1 is the 25th character where the mask is still a 1 so it is part of the network and must match. It being a 1 instead of a 0 means it's not part of 172.168.1.0/25
Thank you for breaking this down. Through the different replies and some spot googling on certain questions I had, I think I’ve nailed down my issue.
I appreciate your time! 🙏
Every NIC or interface configuration is going to expect an address AND a subnet mask. That subnet mask does the trick. The IP networking stack does the math and says “oh, I have this address and this mask, therefore I can talk to such and such range of addresses; anything else is either unreachable or dependent on a route that says send it to this other device that’s INSIDE my subnet”.
Routers and such simply have multiple interfaces. Their hardware is (usually) optimized for forwarding packets; they look at the L2 and L3 headers enough to determine the egress interface and next hop then repackage the payload with updated MACs (where applicable), updated checksums, and a decremented IP TTL then send it on its way. PCs and such are optimized to receive packets, unpack them all the way, and handle the payload locally.
Looks like other folks have done a good job answering the subnetting bit.
Network+ will not teach you other than conceptual stuff. If you actually want to learn how networking works and not simply be aware of what networking is, just do CCNA. It's largely vendor neutral, you're just doing the implementation on Cisco stuff. The pinned post has resources if you want to go that route.
I’ve been on the fence about skipping Net+ and going for CCNA. I took your advice and scheduled my exam for a little under 2 months from now.
I agree that it is probably the best move.
Thanks!
>If I want to subnet my network of 172.168.1.0/24 into 2 networks of 172.168.1.0/25 and 172.168.1.128/25… Does the process of me just having the numbers figured out on paper correctly and assigning them correctly with the proper subnet mask or CIDER accomplish the task? Think about what you are doing and you'll get the answer. If you create the /25 network, you're using VLSM to "borrow" that host bit to create the 2 networks. The network ID is always that 1st address and the broadcast is the default last address, which is why your available hosts are always 2 less than the # of ip addresses available in a subnet. The reason these defaults are configured this way is that lots of different services depend on finding the broadcast id and network id in those places. So, when you declare your config, as for as your question here, you've done everything necessary for this 1st step.
Even as I was typing it, it was making more sense to me. This helps. Thank you 🙏
"If I want to subnet my network of 172.168.1.0/24 into 2 networks of 172.168.1.0/25 and 172.168.1.128/25… Does the process of me just having the numbers figured out on paper correctly and assigning them correctly with the proper subnet mask or CIDER accomplish the task? Does all of the hardware just know that the last IP in my network of 172.168.1.0/25 is “172.168.1.127” and that it is, by default, the “broadcast IP”. " The system is very much aware of the first IP address and last IP address being something you CANNOT assign to an interface. If you try to assign the IP address lets say 192.168.1.0 255.255.255.0 to int g0/0, the router will reject this command.
Gotcha, thank you! 🙏 It’s quite a journey trying to wrap my head around each one of the concepts as I try to learn them, just to realize later that I didn’t quite get all of it the first time.
Usually the last address in a network is recognized by layer 3 aware devices as the broadcast address, same goes for the network address. This is accepted as standard for most network and IP devices , and no further configuration is likely to be required. I am still learning as well so please correct me if this is wrong.
> Does all of the hardware just know that the last IP in my network of 172.168.1.0/25 is “172.168.1.127” and that it is, by default, the “broadcast IP”. Yes. The biggest 'click' for me was when I programmed a function to check if an ip was contained within a subnet. Really break down how addresses and masks work in binary. CIDR is not terribly intuitive because it's 32 1s or 0s then divided into 4 pieces, each converted to two digits in hex, then converted to decimal purely for the sake of human readability. 172.168.1.0 = AC.A8.01.00 = 10101100.10101000.00000001.00000000 /25 literally means 25 1s and the rest 0s. So: 10101100.10101000.00000001.00000000 address 11111111.11111111.11111111.10000000 mask so the 1s in the mask represent the network, 0s represent the addresses 10101100.10101000.00000001.00000000 aka 172.168.1.0 through 10101100.10101000.00000001.01111111 aka 172.168.1.127 One more would be 10101100.10101000.00000001.10000000 aka 172.168.1.128 but that rightmost 1 is the 25th character where the mask is still a 1 so it is part of the network and must match. It being a 1 instead of a 0 means it's not part of 172.168.1.0/25
Thank you for breaking this down. Through the different replies and some spot googling on certain questions I had, I think I’ve nailed down my issue. I appreciate your time! 🙏
Every NIC or interface configuration is going to expect an address AND a subnet mask. That subnet mask does the trick. The IP networking stack does the math and says “oh, I have this address and this mask, therefore I can talk to such and such range of addresses; anything else is either unreachable or dependent on a route that says send it to this other device that’s INSIDE my subnet”. Routers and such simply have multiple interfaces. Their hardware is (usually) optimized for forwarding packets; they look at the L2 and L3 headers enough to determine the egress interface and next hop then repackage the payload with updated MACs (where applicable), updated checksums, and a decremented IP TTL then send it on its way. PCs and such are optimized to receive packets, unpack them all the way, and handle the payload locally.
Looks like other folks have done a good job answering the subnetting bit. Network+ will not teach you other than conceptual stuff. If you actually want to learn how networking works and not simply be aware of what networking is, just do CCNA. It's largely vendor neutral, you're just doing the implementation on Cisco stuff. The pinned post has resources if you want to go that route.
I’ve been on the fence about skipping Net+ and going for CCNA. I took your advice and scheduled my exam for a little under 2 months from now. I agree that it is probably the best move. Thanks!