I am really struggling on learning how to subnet. This is no joke, I have been beating this stuff in my head for 2 days straight and I am no closer now than I was when I first started this chapter. Here is one example if someone could spoon feed me the answer I would appreciate it. I have an IP address of 138.43.39.152/29. I am trying to find the Network ID, the broadcast ID, the first usable host and the last usable host. Here is what I have done and I realize it is wrong, but this is where I am stuck.
138.43.39.15
255.255.255.240
The subnet in binary form breks down to:
11111111 11111111 11111111 11111000
so I take the 0 bits and come up with 2 to the 3rd -2 which is 6 hosts per subnet. so I have the ip ranges as follows:
138.43.39.0 - 138.43.39.5
138.43.39.6 - 138.43.39.11
I keep doing this all the way to the 138.43.39.150 - 138.43.39.155 range as the original IP of 138.43.39.152 falls into this range. So I come up with the following:
138.43.39.150 as the network ID
138.43.39.155 as the Broadcast
138.43.39.151 - 138.43.39.154 as the available ip range This leaves me with 4 addresses to use + the 2 ip's for the network and broadcast which gives me the 6 hosts in that range.
Please explain to me where I am going wrong!!!!!
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Ps- I recommend learning to do this long hand as it helps to understand, but a subnet calculator such as in the following link may be helpful to check your work:
http://jodies.de/ipcalc?ho
RobWill: Good catch on the subnet being incorrect. I still have a few questions here.
This is my understanding. 2^3=8 then you subtract 2. One for the network ID and the other for the Broadcast. So your saying I should be including 8 ip's in each subnet, but just keep in mind that 2 of the 8 are unusable? So if this is correct then I should be doing:
138.43.39.0 - 138.43.39.7
138.43.39.8 - 138.43.39.15
138.43.39.16 - 138.43.39.23
etc. etc. etc.
Correct!
Just keep in mind it is base 2, so everything will be in increments of 2 exponent 'x', i.e. 2, 4, 8, 16, 256
Same applies with bigger subnets like:
192.168.0.0/24
255.255.255.0 is the subnet mask
192.168.0.0 is the network ID
192.168.0.1 to 192.168..0.254 are usable IP's
192.168.0.255 is the broadcast IP
2^8-2 (i.e. 254 usable IP's and 1 NetID and 1 Broadcast)
Here is the quick down and dirty on how I teach my Marines how to subnet.
/24 /25 /26 /27 /28 /29 /30 /31 /32
255 128 64 32 16 8 4 2 1
1 2 4 8 16 32 64 128 255
The Top line represents the CIDR notation of the network or how many bits are "turned on" for that network.
R/S
SSgt Lee Chieffalo
Network Engineer USMC
The middle line is how many valid IP's you can have with a network that has that subnet mask. For example a /27 network has 32 valid IP's in that network. Remember with IP's you always have to subtract 2 for the network ID & broadcast.
The last row is how many valid networks you can have with that subnet if you use traditional subnetting or basically no VLSM. For example you can break up a class C network into 8 /27's.
Also all valid starting points for these networks have to be divisible by the number in the middle row. For example if you wanted to use a /27 somewhere in the middle of your network you would have to start it at a IP that is divisible by 32. For example valid starting points are 0, 32, 64, 96, 128, 160, 192, & 224.
If you need to go into supernetting such as Class B net's just replace the top row with /16 - /23 and the middle 2 rows would represent whole class C networks.
Hi,
138.43.39.152/29
steps
1)write down /29 in binary that is 11111111.11111111.11111111
2)find the increment for /29 or 255.255.255.248 correction it is not 240
3)increment is 8, this is binary 1st starting binary 1 from right side. that is
128 64 32 16 8 4 2 1 this is in fourth octet. so your subneting is in fourth octet.
1 1 1 1 1 0 0 0
4)now write down the network and subnet.
138.43.39.0
138.43.39.8
138.43.39.16
.24
.32
.40
.48
.56
.64
.72
.80
.88
.96
.104
.112
.120
.128
.136
.144
138.43.39.152 ------138.43.39.159 this is your network range.. in this 152 is network id and 159 is broadcast id
.160 and in between is usable ip i.e. from 153 to 158.
i hope this is usefull and clear for you...go step by step..practice as much as possible with subnets.
Regards,
Praveen
Forget the bitwise operations. Here's how I do subnets. In my head! Any subnet, any mask in 30 seconds or less.
1. First - credit where it is due. My method has been shamelessly stolen from Wendell Odom's CCENT/CCNA ICND1 Official Exam Certification Guide.
2. The whole secret around subnets lies in:
a) the subnet mask
b) knowing your 2x, 4x, 8x, 16x, 32x, 64x and 128x times tables as far as 256
Most people can handle 2x, 4x and (at a pinch) 8x. 32x, 64x and 128x only have 8, 4 or 2 items before you hit 256, so that leaves you your 16x tables to learn.
Here's the deal:
Look at the "interesting" number in the subnet mask. That is, the number that is NOT 0 or 255. In your case:
138.43.39.15
255.255.2
Your
Subtract this number from 256 - you'll get 16 (256-260=16) in this case. If the mask was 255.255.255.224 you'd get 256-224=32. There are only 7 possibilities (excluding 0 and 255). Learn them. I'll list them later for you.
Back to 138.43.39.15 255.255.255.240
256-240=16.
The key to understanding subnet is this "magic number". Read the following carefully:
If the "magic number" is 16, then
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by: MCSA2003Posted on 2009-10-13 at 20:27:34ID: 25567007
Correction on the IP address I have listed after the first paragraph. It reads 138.43.39.15 and should be 138.43.39.152