Is length of ethernet header necessarily 14?

Question

Code snippet from here:

void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data)
{
    ....
    /* retireve the position of the ip header */
    ih = (ip_header *) (pkt_data +
        14); //length of ethernet header
    ....

But this image doesn't say it's necessarily 14:

How should I do it properly?

Answer 1

In 802.3, both the source and destination addresses are 48-bit MAC addresses. 6+6+2=14

Answer 2

The Wikipedia has a good picture of the frame

WIKI

IPv4 / v6 are layer 3 protocols.

Answer 3

The ethernet header is fixed width however extension protocols such as 802.1q for vlan/qos are common and effectivly extend the L2 header.