Calculate subnet bits and host bits

0
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An organization is given an IP address block of 130.34.12.128/27. The organization must have 5 different subnets.

  1. What is the number of subnet bits and host bits?

  2. How many hosts are available per subnet?

  3. What are the subnet addresses and range of addresses for each subnet?

  4. What is the new subnet mask value?

I google and found out that the organization must only have the value of power 2. So the subnets is it must be 8? May I know how does this works?

2
hostipv4subnet

2 Answers

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As you may know, networks have adresses shorter than 32 bits ended by trailing zeroes. In your case, the org have a 27 bit adress ended by .128 which is .10000000 in binary, among which only .100 serves to identify which network. The last 5 bits are trailing zeroes and can be used to identify either subnets or machines inside the network.

This means a machine can have an adress 130.34.12.129/32 it will have a public adress inside the network (same first 27 bits but ending by 00001). With only machines, this would be 2^5 - 2 = 30 distinct hosts in the total network (we remove the adress 00000 as its the net adress and 11111 as its the broadcast adress). The adress range would be 130.34.12.129 to 130.34.12.159

Now if you need 5 subnets, this means you have to find five addresses ended by trailing zeroes inside the original net address. An example would be 00 000 01 000 10 000 111 00 110 00. If you have understood you should be able to answer the 4 questions for each of these subnets (number of hosts, mask etc.)

0
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You have address 130.34.12.128 with mask 255.255.255.224. In binary you have:

10000010 00100010 00001100 100 00000
11111111 11111111 11111111 111 00000

You can change only the last 5 binary so you have from 130.34.12.128 to 130.34.12.159 ip address. All subnet need 1 addres for brodcast and 1 for that subnet address.

You can make example:

130.34.12.128 - 130.34.12.135 /29 6 ip address + 1 brodcast + 1 subnet address
130.34.12.136 - 130.34.12.143 /29 6 ip address + 1 brodcast + 1 subnet address
130.34.12.144 - 130.34.12.151 /29 6 ip address + 1 brodcast + 1 subnet address
130.34.12.152 - 130.34.12.155 /30 2 ip address + 1 brodcast + 1 subnet address
130.34.12.156 - 130.34.12.159 /30 2 ip address + 1 brodcast + 1 subnet address

In binary last octet:

10000000 - 10000111 /29
10001000 - 10001111 /29 
10010000 - 10010111 /29 
10011000 - 10011011 /30
10011100 - 10011111 /30

You can see on the last subnet we can't make more ip address, mask show you what you can change, so every your company ip addres will start from 10000010 00100010 00001100 100 and you can changed only the last 5 "numbers". 

  1   1   1  1    1   1   1   1
128  64  32  16   8   4   2   1



10000010 00100010 00001100 10000000
 128 + 2  32 + 2    8 + 4    128
    130     34       12      128