check if an IP is a member of a subnet.

ip_in_subnet.lua

-- I only found overcomplicated solutions for this small problem so I wrote this for myself.
-- github.com/ZsBT


-- convert IP string to long integer
local function ip2long( ip )
    if not ip then return false end
    local o1,o2,o3,o4=ip:match("^(%d%d?%d?)%.(%d%d?%d?)%.(%d%d?%d?)%.(%d%d?%d?)$")
    if not o1 or not o2 or not o3 or not o4 then return false end
    return tonumber(o4) + (tonumber(o3) << 8) + (tonumber(o2) << 16) + (tonumber(o1) << 24)
end


-- check if ip (string) is in the given subnet (string)
local function ipinnet( ip, subnet )
    local ipL = ip2long(ip)
    local range, netmask = subnet:match("^(%d%d?%d?%.%d%d?%d?%.%d%d?%d?%.%d%d?%d?)/(%d%d?)$")

    if not range then
        range = subnet:match("^(%d%d?%d?%.%d%d?%d?%.%d%d?%d?%.%d%d?%d?)$")
        netmask = 32
    end

    if not range then return nil end

    rangeL = ip2long(range)
    wildcardL = ( 2 ^ ( 32 - tonumber(netmask) ) >> 0 ) - 1
    netmaskL = ~ wildcardL

    return ( ( ipL & netmaskL) == ( rangeL & netmaskL) ) 
end

print ( ipinnet( "10.252.3.1",  "10.252.3.1") )
print ( ipinnet( "11.252.3.1",  "10.252.3.1/15") )
print ( ipinnet( "127.10.5.5",  "127.0.0.0/8") )