Obtain IP address from user input, generates 3 other static IPs - powershell

Question

I am looking for a more efficient way to obtain an IP address from a user, splitting the IP address into an array, knocking off the last octet, replacing with a specific number, then joining the array back into a new IP address.
For example: User inputs IP: 10.1.1.50. From this input, I take the first 3 octets, then generate 10.1.1.1, 10.1.1.10 and 10.1.1.11. I can procedurally do this, but seems Im doing the same operations over for 3 iterations, a function could possibly be beneficial, but my attempts are failing.
$ip= $(read-host "Enter any IP Address")
$gateway = $ip.split('.')
$gateway[-1] = 1
$gateway = $ipArray -join '.'
$dns1 = $ip.split('.')
$dns1[-1] = 10
$dns1 = $dns1 -join '.'
$dns2 = $ip.split('.')
$dns2[-1] = 11
$dns2 = $dns2 -join '.'
I've separated out the first 3 octets with $threeOctets = $ip | Select-Object -First 3, which works fine, what any time i operate on it, i just seem to ADD on the 1, then the 10, then the 11 instead of adding on the 1, then going back to the first 3 octets and adding a 10, then going back and adding an 11.

Answer 1

Why are you making the user type in the last octet, only to cut it off and throw it away? And are you sure this is a good design, given that networks aren't all /24?
Anyway, no real need to split and join, you can just:
$ip = (Read-Host "Enter an IP") -replace '\d+$'
$gateway, $dns1, $dns2 = ('1','10','11' | foreach {"$ip$_"})
The replace gets rid of the last digits. The loop puts a string into the pipeline for each of the three numbers in the array. The multiple assignment takes the three values coming out of the pipeline into the three variables.
(PowerShell does have rudimentary IP address types, with [ipaddress]"192.168.0.1", but they're too basic to be of much help here).

Answer 2

I agree with was TessellatingHeckler's answer says. But I wanted to show another way that has some basic error correction as well using [ipaddress].
do{
$ip = $(read-host "Enter any IP Address")
} while(!($ip -as [ipaddress]))
$octets = ([ipaddress]$ip).Getaddressbytes()
$gateway, $dns1, $dns2 = '1','10','11' | ForEach-Object{$octets[3] = $_; $octets -join "."}
This is by no means a better approach. Merely an alternate one. The error correction is not perfect and can break but its better than a kick in the pants in some cases.
Get the ip. Convert it into its octets as an array. replace the last element of the array with the pipeline value. Join again with a period to make it an valid ip again.
It is a shame that the [Ipaddress] class wont let you edit the octets in place. [version] works like this too, which is unfortunate.