hi
can you help me with this
i have a network of 3 routers
he number of hosts on each network
Net1 60
Net2 90
Net3 60
Net4 30
Net5 60
Net6 128

there are 6 Pcs

how can i calculate the subnet mask and knowing the subnet ,default getway,,,

All 2 Replies

hi
can you help me with this
i have a network of 3 routers
he number of hosts on each network
Net1 60
Net2 90
Net3 60
Net4 30
Net5 60
Net6 128

there are 6 Pcs

how can i calculate the subnet mask and knowing the subnet ,default getway,,,

ID # Bits Number Hosts Usable Hosts Netmask Note
6 /6 67108864 67108862 252.0.0.0
7 /7 33554432 33554430 254.0.0.0
8 /8 16777216 16777214 255.0.0.0 Class A Network
9 /9 8388608 8388606 255.128.0.0
10 /10 4194304 4194302 255.192.0.0
11 /11 2097152 2097150 255.224.0.0
12 /12 1048576 1048574 255.240.0.0
13 /13 524288 524286 255.248.0.0
14 /14 262144 262142 255.252.0.0
15 /15 131072 131070 255.254.0.0
16 /16 65536 65534 255.255.0.0 Class B Network
17 /17 32768 32766 255.255.128.0
18 /18 16384 16382 255.255.192.0
19 /19 8192 8190 255.255.224.0
20 /20 4096 4094 255.255.240.0
21 /21 2048 2046 255.255.248.0
22 /22 1024 1022 255.255.252.0
23 /23 512 510 255.255.254.0
24 /24 256 254 255.255.255.0 Class C Network
25 /25 128 126 255.255.255.128
26 /26 64 62 255.255.255.192
27 /27 32 30 255.255.255.224
28 /28 16 14 255.255.255.240
29 /29 8 6 255.255.255.248
30 /30 4 2 255.255.255.252

I am trying to attach this as a word doc also...

how can i calculate the subnet mask and knowing the subnet ,default getway,,,

The value 256 is a "magic number" in networking and IPV4 subnet calculations. A few simple formulas will let you easily calculate any requirement. Here's a set of formulas that I give my students when I teach networking classes:

Subtract the netmask value from 256 to find the number of hosts or host groups per subnet. If there are ZERO values following the mask octet, multiply the result by 256 for each following zero.
mask=255.255.255.192, 256-192=64, less 2 yields 62 hosts
mask=255.255.240.0, 256-240=16, times 256=4096, less 2 yields 4094 hosts
mask=255.252.0.0, 256-252=4, times 256=1024, times 256=262144, less 2 yields 262142

In example 2 - a class B netmask - any zero values to the right of the subnet value represent groups of 256 addresses. You must find the number of address groups, then multiply by 256 for each zero to the right. Example 3 shows a class A subnet.

Divide 256 by the number of subnets needed to determine the # of hosts per subnet
Need 8 subnets - divide 256 by 8 to get 32 hosts per subnet.

Subtract the number of hosts from 256 to determine the netmask.
Need 4 subnets - divide 256 by 4 - yields 64 hosts per subnet. 256-64=192, netmask is 255.255.255.192

Defining Subnets
1. Determine number of subnets needed (7)
2. If first/last subnets are not allowed, add 2 to the number of subnets needed (7+2=9) This is usually NOT "real-world" but is the way MS used to teach IP, so you might need to do this for exams.
3. Round up to the next power of 2 (9 => 16)
4. Divide 256 by the number of subnets to determine # of hosts / subnet (256 / 16 = 16)
5. Subtract result from 256 (256 - 16 = 240)
6. Subtract 2 from the number of hosts to find the number of valid host addresses (16 - 2 = 14)
7. Make sure that the subnets are large enough to accomodate the network hosts
Request a larger address space if necessary