verifone411
join:2008-05-08
| How do I determine subnet manually? This is more of a question of how than what. Since there are calculators on how to do this on the internet.
If one has been assigned a Class B address (141.67.x.x), and you need to be able to designate up to 4,000 hosts per subnet. What subnet mask specification would allow for the most possible subnets? How did you determine your subnet mask specification?
Excluding reserved addresses, how many addresses would be available for host assignment in each subnet? In total for all subnets?
What would be the last assignable address of the first subnet, and what would be first assignable address of the last subnet? | |
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koitsu
Premium
join:2002-07-16
Mountain View, CA
1 edit | Re: How do I determine subnet manually? I think this was posted to the wrong forum. Should be over in one of the Networking or Advanced Networking forums. 
Regardless, an attempt answer your question -- and it's hard to answer because I'm not completely sure I understand the question fully:
You'll want to split 141.67.0.0/16 into separate subnets that offer enough IP space to guarantee at least 4000 (usable) IPs per block. That's fairly easy to figure out. Let's start with a /24 (what you call a "Class C") and break it down from there:
/24 = 256 IPs
/23 = 512 IPs
/22 = 1024 IPs
/21 = 2048 IPs
/20 = 4096 IPs
So what you'd have is multiple /20 subnets, guaranteeing 4093 usable IPs out of each of those netblocks (lost 3 IPs go to gateway, broadcast, and network).
How many of those subnets would you have? Well, that's simple: A /16 gives you 65536 IPs, so do some simple math:
65536 / 4096 = 16
You'd have a total of 16 /20 netblocks to use. Thus, your segregated netblocks would be:
141.67.[[16*0 through 16*15]].0/20, e.g.:
141.67.0.0/20
141.67.16.0/20
141.67.32.0/20
141.67.48.0/20
141.67.64.0/20
141.67.80.0/20
141.67.96.0/20
141.67.112.0/20
141.67.128.0/20
141.67.144.0/20
141.67.160.0/20
141.67.176.0/20
141.67.192.0/20
141.67.208.0/20
141.67.224.0/20
141.67.248.0/20
Now, you lose 3 IPs per /20 (one for network, one for broadcast, and one for gateway [optional]). So ultimately you're "losing" 48 IPs due to the subnetting of the /16.
Hope this makes sense.
--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer. | |
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IamGimli
join:2004-02-28
Canada
 ·Primus Talkbroadband
 ·B2B2C High-Speed I..
| Since you asked, here's the "how".
A subnet mask determines how many bits of your IP address are static for a particular subnet.
Let's use your class B as an example. 141.67.0.0 is represented as 10001101.01000011.00000000.0000000 in binary. The subnet mask of your class B is /16, because the first 16 bits of that address will never change. When you want to further separate that class B in smaller subnet you remove more bits that will not change for that subnet.
For example, koitsu's suggestion of /20 subnets to get the most subnets out of your class B while having at least 4000 addresses in each subnet makes your first subnet 10001101.01000011.0000xxxx.xxxxxxxx in binary. That means that subnet includes every addresses between 10001101.01000011.00000000.00000000 and 10001101.01000011.00001111.11111111 (141.67.0.0 to 141.67.15.255). Your second subnet would be 10001101.01000011.0001xxxx.xxxxxxxx (141.67.16.0 to 141.67.31.255), your third 10001101.01000011.0010xxxx.xxxxxxxx (141.67.32.0 to 141.67.47.255), etc.
The x represent every variable bit in the addresses for that subnet.
If you went with a /22 subnet mask instead, the mask would look like 10001101.01000011.000000xx.xxxxxxxx and the addressable range in your first subnet would be 10001101.01000011.00000000.00000000 to 10001101.01000011.00000011.11111111 (141.67.0.0 to 141.67.3.255). | |
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Bobthebuilder
@instantnetworks.net | Re: How do I determine subnet manually? 141.67.0.0 is represented as 10001101.01000011.00000000.0000000
How do you convert to binary without a calculator that I see all over the net? | |
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| | IamGimli
join:2004-02-28
Canada
·Primus Talkbroadband
·B2B2C High-Speed I..
3 edits | Re: How do I determine subnet manually? In binary, each position is an exponent of 2 (instead of an exponent of 10 in the decimal system). The first position is therefore worth 1 (2^0), the second position is worth 2 (2^1), the third 4 (2^2), etc. From that it's a bit of mental work to determine the whole number. For 141, for example, what whole exponents of 2 are represented? You got 128 (2^8) or 10000000 in binary, plus 8 (2^4) or 1000, plus 4 (2^3) or 100, plus 1 (2^0) or 1.
bin 10000000 + 1000 + 100 + 1 = 10001101
dec 128 + 8 + 4 + 1 = 141
hex 80 + 8 + 4 + 1 = 8D (disregard that, I'm just trying to confuse you with hexadecimal i.e. base 16 now  )
BTW even the Windows calculator can easily translate from decimal to binary values. Just switch it to scientific mode and you'll see the Hex, Dec, Oct and Bin radio button on the left side. Enter a number while the radio button is on Dec, click on the Bin button and see it represented in binary. Just remember to add the leading zero(s) if your value is less than 128 and you are representing an IP address. Each of the four bytes in the IP address needs to be represented by 8 binary positions or bits (hence why you represent 67 with 01000011 instead of just 1000011). | |
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| | | verifone411
join:2008-05-08
| Re: How do I determine subnet manually? Thanks for the infomation. Sticking with the first class b number (does that change things since you decided to move the example to a class c?)
How do you know and how do you determine that...
/24 = 256 IPs = 254 Usable ? i know the diagrams say that.
Is it because the last 2 sets of octects represent 8 bits each for a total of 16. Each 16 bit placeholder can have 2 different values so that is why you raise it to the power of 2? to get 65536- 2=65534 usable? | |
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| | | | | IamGimli
join:2004-02-28
Canada
·Primus Talkbroadband
·B2B2C High-Speed I..
| Re: How do I determine subnet manually? said by tschmidt  :If you stick with binary or hex it is a lot less confusing. I understand binary but how is hexadecimal any less confusing than decimal to represent network addresses? It may make them a bit shorter to write but I don't see any difference in the confusion aspect. | |
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verifone411
join:2008-05-08 | (one for network, one for broadcast, and one for gateway [optional]).
I am guessing...
network = .0
broadcast= .255
gateway =.1
?
thanks | |
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