Confused on a subnetting question....
no!all!
Member Posts: 245 ■■■□□□□□□□
in CCNA & CCENT
So, I've been using subnettingquestions.com and I came across one that I'm stuck on...
Q: How many subnets and hosts per subnet can you get from the network 172.26.0.0 255.255.255.128?
A: 512 Subnets and 126 Hosts
I got 2 subnets and 126 hosts...where does 512 come from?!
Q: How many subnets and hosts per subnet can you get from the network 172.26.0.0 255.255.255.128?
A: 512 Subnets and 126 Hosts
I got 2 subnets and 126 hosts...where does 512 come from?!
A+, N+, S+, CCNA:RS, CCNA:Sec
"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin
2019 Goals: CCNP:RS & relocate to St. Pete, FL!
"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin
2019 Goals: CCNP:RS & relocate to St. Pete, FL!
Comments
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no!all!
Member Posts: 245 ■■■□□□□□□□
Yes, but I though you could only use the last octet for the two networks 0 and 128...which is where I got 2 subnets...A+, N+, S+, CCNA:RS, CCNA:Sec
"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin
2019 Goals: CCNP:RS & relocate to St. Pete, FL! -
Xyro
Member Posts: 623
No, the last octet is split 1-7
172.26.0.0 255.255.255.128
xxxxxxxx.xxxxxxxx.12345678.9xxxxxxx = 9 subnet bits
xxxxxxxx.xxxxxxxx.xxxxxxxx.x1234567= 7 host bits (-2) -
no!all!
Member Posts: 245 ■■■□□□□□□□
I'm confused now!
Agreed, here was my math...
172.26.0.0
255.255.255.128
256-128 = 128 which would be 0 and 128...or 2 subnets...right?! Unless there's something special about the 3rd and 4th octet being 0's that I just don't realize...Still don't see where 512 subnets is coming from
172.26.0.0 and 172.26.0.128A+, N+, S+, CCNA:RS, CCNA:Sec
"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin
2019 Goals: CCNP:RS & relocate to St. Pete, FL! -
no!all!
Member Posts: 245 ■■■□□□□□□□
Class B - default subnet mask ... so begin in 3rd octet
Wait, I think I see it....hang on..A+, N+, S+, CCNA:RS, CCNA:Sec
"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin
2019 Goals: CCNP:RS & relocate to St. Pete, FL! -
prdemon
Member Posts: 54 ■■■□□□□□□□
the third octet in your network address is 0 and your subnetmask in the third octet is 255.
that means all 8 bits are for the network and you borrowed one bit from the last octet , thats where your 128 is comibg from -
HAMP
Member Posts: 163
Agreed, here was my math...
172.26.0.0
255.255.255.128
256-128 = 128 which would be 0 and 128...or 2 subnets...right?! Unless there's something special about the 3rd and 4th octet being 0's that I just don't realize...Still don't see where 512 subnets is coming from
172.26.0.0 and 172.26.0.128
That is how I have known it to work. It would seem I am missing something also. -
FloOz
Member Posts: 1,614 ■■■■□□□□□□
You guys that are confused are looking at the network as class C. Remember the default mask for 172.16.0.0/16 is Class B. -
xnx
Member Posts: 464 ■■■□□□□□□□
So, I've been using subnettingquestions.com and I came across one that I'm stuck on...
Q: How many subnets and hosts per subnet can you get from the network 172.26.0.0 255.255.255.128?
A: 512 Subnets and 126 Hosts
I got 2 subnets and 126 hosts...where does 512 come from?!
Class B Address: /16 Default (255.255.0.0)
3rd octet = 255 = 8 bits
4th octet = 128 = 1 bit
9 bits total for number of subnets
2^9 = 512
7 bits left from the 16 for hosts
(2^7) -2 = 126 hosts per subnetGetting There ...
Lab Equipment: Using Cisco CSRs and 4 Switches currently -
HAMP
Member Posts: 163
the third octet in your network address is 0 and your subnetmask in the third octet is 255.
that means all 8 bits are for the network and you borrowed one bit from the last octet , thats where your 128 is comibg from
I believe I need to revisit subnetting. I don’t remember a formula when the network is ‘0’, while the mask is ‘255’. I didn’t know that made a difference. I am so glad I found this site!!!!!
I was using the same formula that “no!all!” is using -
xnx
Member Posts: 464 ■■■□□□□□□□
No worries, subnetting can be tricky when you start learning, I can only do it easily because I learnt binary and hex many years agoI believe I need to revisit subnetting. I don’t remember a formula when the network is ‘0’, while the mask is ‘255’. I didn’t know that made a difference. I am so glad I found this site!!!!!
I was using the same formula that “no!all!” is usingGetting There ...
Lab Equipment: Using Cisco CSRs and 4 Switches currently -
no!all!
Member Posts: 245 ■■■□□□□□□□
Yea, HAMP, you and I were thinking the same thing....thanks xnx for the info!A+, N+, S+, CCNA:RS, CCNA:Sec
"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin
2019 Goals: CCNP:RS & relocate to St. Pete, FL! -
HAMP
Member Posts: 163
I went to the subnetting site to do a few questions, and I came across this one:
What valid host range is the IP address 172.24.89.220 255.255.255.224 a part of?
I gotten the correct answer, and I know how I did it.
172.24.89.193 through to 172.24.89.222
To me, I see having 8 subnets with 30 usable host each subnet (240 host)
I don't see 2048 subnets with, which I gotten from the formula you guys just gave. -
omi2123
Member Posts: 189
even though they used a class C subnet mask, but by default it's a class B network & the subnet counts from the third octet......512 subnet is correct... -
Xyro
Member Posts: 623
What valid host range is the IP address 172.24.89.220 255.255.255.224 a part of?
To me, I see having 8 subnets with 30 usable host each subnet (240 host)
I don't see 2048 subnets with, which I gotten from the formula you guys just gave.
No, there are 11 subnets bits here. Again, it is a Class B you are working with.
xxxxxxxx.xxxxxxxx.12345678.123xxxxxx = 11 Subnet bits
xxxxxxxx.xxxxxxxx.xxxxxxxx.xxx12345 = 5 Host bits
11 Subnet bits = 2048
5 host bits = 32 (-2) -
OfWolfAndMan
Member Posts: 923 ■■■■□□□□□□
Just remember these numbers:
0-126 (First octet)= Class A. 255.0.0.0 Subnet mask
127 is a loopbavk address
128-191=Class B (255.255.0.0 default subnet mask)
192-223=Class C (255.255.255.0 subnet)
224 and up is generally multicast
After assessing class, look at how many bits they added. 172 is class B, therefore having 255 in third octet is 8 added net bits, along with the other one in the fourth octet. That leaves seven host bits. Subnets = 2^9 Hosts total = 2^7-2 (Which excluded network and broadcast):study:Reading: Lab Books, Ansible Documentation, Python Cookbook 2018 Goals: More Ansible/Python work for Automation, IPSpace Automation Course [X], Build Jenkins Framework for Network Automation [] -
HAMP
Member Posts: 163
I see what my problem is, but it’s still confusing. As what FloOz and omi2123 mentioned, it is a class B and not a class C. I’ve been looking at the mask and using a formula based on the mask classes and not the IP addy class. (did I say that right? lol). That seems to be a different formula that I not able to do In my head just yet.
-
OfWolfAndMan
Member Posts: 923 ■■■■□□□□□□
The ones that aren't already subnetted are pretty simple once you get used to the formula. It's subnetting the IP addresses that are already subnetted that make it a bit more fun
You'll really want to know your subnetting as well when you start summarizing addresses in OSPF/EIGRP for your ABRs/ASBRs. :study:Reading: Lab Books, Ansible Documentation, Python Cookbook 2018 Goals: More Ansible/Python work for Automation, IPSpace Automation Course [X], Build Jenkins Framework for Network Automation []
