all this solutions are needed, because the as-command:
IPv4 address of (system info)
only returns "missing value"
the reason for this is the virtual IP given from my router

now i found another solution for this problem:
last word of (do shell script "system_profiler -detailLevel basic | grep 'IPv4 Addresses'")
i think, this is the most dependable way, but also the slowest.

@NovaScotian (Adam Bell!): Thanx for your permission