import ipaddress


def is_subnet_of(a, b):
    """
    Returns boolean: is `a` a subnet of `b`?
    """
    a = ipaddress.ip_network(a)
    b = ipaddress.ip_network(b)
    a_len = a.prefixlen
    b_len = b.prefixlen
    return a_len >= b_len and a.supernet(a_len - b_len) == b