Addressing on WAN Serial Links
One major difference between Ethernet and HDLC or PPP lies in
the address field inside the HDLC and PPP header. There's only one address
field, and it's 1 byte long. However, if you sit back and think about it, why do
you need an address at all? When R1 sends anything on this point-to-point WAN
link, the only device that could possibly get the data is R2. So, although the
address field exists, it doesn't really matter. Moreover, most people don't
think about the existence of an HDLC or PPP address field in the header.
A convenient side effect of the boring address field for HDLC
and PPP is that there is no need for Address Resolution Protocol (ARP) or
anything like it on the serial link. You might recall from Chapter 11 that when R1 forwarded an IP packet to R2
over a LAN, R1 had to use ARP to learn R2's data link (Ethernet) MAC address.
With point-to-point WAN data link protocols, R1 doesn't need to find R2's HDLC
address because the frame will be delivered to R2 over the link anyway. It's the
only place the frame could go!
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