Member Avatar for ITgirl2010

This is a question i got and i just dont under stand how to do it.

An organization is assigned the network number and it must create a set of subnets that supports up to 90 hosts on each subnet.

How do you Define subnet mask for this organization

What is the IP number for the Subnet number 421 explicitly in binary and decimal notation

What is the explicitly IP address of the host number 68 on the Subnet number 421


What is the broadcast address for the Subnet number 421

Any guideience would be great cause im stressing out :(

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Generated

Short, practical method (exam-friendly) and the indexing pitfall that caused the confusion in the thread.

Find the mask

  • Let H = required usable hosts. Compute b = ceil(log2(H + 2)). The subnet mask length is 32 − b and each subnet has 2^b addresses (2^b − 2 usable hosts). For H = 90 this gives a /25 mask (128 addresses, 126 usable).

Map a subnet number to an address

  • Work in 32‑bit integers: subnet_size = 2^b. Zero‑based subnet index k = subnet_number − 1 (if the problem uses one‑based numbering use k = N − 1; if it uses zero‑based use k = N). Network_int = base_network_int + k * subnet_size. Broadcast_int = Network_int + subnet_size − 1. Host#h (the hth usable host) = Network_int + h (if h counts the first usable host as 1).

Apply to the given question (both common conventions)

  • If subnet numbering is one‑based (first subnet = base network), subnet 421 → 172.18.210.0/25. Host #68 on that subnet → 172.18.210.68. Broadcast → 172.18.210.127.
    • Binary examples: network = 10101100.00010010.11010010.00000000; host = 10101100.00010010.11010010.01000100; broadcast = 10101100.00010010.11010010.01111111.
  • If the question used zero‑based subnet numbers (some people do), subnet 421 → 172.18.210.128/25. Host #68 there → 172.18.210.196. Broadcast → 172.18.210.255.
    • Binary examples: network = 10101100.00010010.11010010.10000000; host = 10101100.00010010.11010010.11000100; broadcast = 10101100.00010010.11010010.11111111.

Notes tied to the thread

  • used the same block size but the thread shows an indexing shift; that is the source of and ’s confusion. Always check whether subnet numbering is zero‑based or one‑based and whether “host number X” means the Xth usable host.

Recommended Answers

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Member Avatar for freshfitz

Where are you coming up with 421? A /16 subnet is 255.255.0.0. The ip would be 172.18.0.whatever

Member Avatar for ITgirl2010

Yes That the part I mstly didn't understand
I assumed that If the subnet number was 421 what would the ip address be

Member Avatar for freshfitz

First off what are you trying to do. I'm assuming connecting an Internet router? Your ip should be 172.18.0. Whatever they gave you. You subnetmask should be Your gateway should be 172.18.0. Whatever the isp assigned.

Member Avatar for Windgator

Here it goes :_)

Bits from the left to right
8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1th bit
65535, 32768, 16384, 8192, 4096, 2048, 1024, 512, 256, 128, 64, 32, 16, 8, 4, 2

We will use VLSM to create such a beautiful class B address with 512 networks,
and 126 host in each one (128 - 2, one-for network address, one-for broadcast
is reserved)

11111111.11111111.00000000.00000000 network bits
00000000.00000000.11111111.11111111 host bits

Using VLSM we will borrow host bits to create subnets with 90 host (126 actually)

0 1 1 1 1 1 1 1 < last octet (4th)
7 6 5 4 3 2 1th bit
128, 64, 32, 16, 8, 4, 2

7 bits will give as more than enough space for hosts. We need 90, we must have at least 126.

1 1 1 1 1 1 1 1 10000000 3rd and 4th octet
8 7 6 5 4 3 2 1 8th bit of the 4th octet
512, 256, 128, 64, 32, 16, 8, 4, 2


Last bit from 4th octet will go to the 3rd octet giving au 512 networks to run for.



So now we have 512 subnets with 126 host each.

network 1th
172.18.0.1-126 hosts
broadcast

network 2nd
172.18.0.129-254 hosts
broadcast

network 3rd
172.18.1.1-126 hosts
broadcast

network 4th
172.18.1.129-254 hosts
broadcast

network 5th
172.18.2.1-126 hosts
broadcast

and so on



network 420th
172.18.210.1-126 hosts
broadcast

network 421 The One :_)
172.18.210.129-254 hosts
broadcast


129 first host + 67 = 196

This is 68th host on the 421th subnet of
network made by Variable Length Subnet Masking (VLSM) from
address giving us 512 networks with 126 host each. (Remember that first host
address is reserved for network, and last for broadcast.)



host
10101100.00010010.11010010.11000100

subnet mask
11111111.11111111.11111111.10000000

[End of File]

Edited by Windgator because: n/a
Member Avatar for Azdy Asiry

Thanks for that but can you describe it by simple way, please???
I need to find the question solution of the close time.

Member Avatar for Casper429

Windgators answer is accurate..
but i have a question on the last part..
How did you come up with the 42st subnet??
Did u use any binary conversion?? (Or is it just by dividing the number of subnets )
If you can please explain this..
Thanks

commented: The last post is last year. Don' t reply to it -1
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