is_ipaddr.py

gistfile1.py

def is_ip4addr(some_string):
    '''True if some_string is an IPv4 address'''

    try:
        octets = [int(o) for o in some_string.split('.')]
    except ValueError: # because some element cannot be cast to integer
        return False

    # ip addresses have between one and four octets
    if len(octets) > 4:
        return False

    # True if all octets fall into 0-255 range
    return all([ o <= 255 and o >=0 for o in octets ])

But seriously, just use netaddr