Clarification on netlink requests

[David Chappelle]

I am a bit confused with respect to the structure of netlink requests.
It seems that in some circumstances a request can look like:

    struct request
    {
        struct nlmsghdr header;
        struct rtgenmsg body;
    };

and in other cases it can look like:

    struct request
    {
        struct nlmsghdr header;
        struct ifinfomsg body;
    };

How do I know which one to use when sending RTM_GETLINK and
RTM_GETADDR requests?