Trying to understand how hdlc works based on this topology that I created to better visualise my question:

enter image description here

I've been reading the Chapter 3 of Wendel Odom book, but not understanding the purpose of the WAN properly.

Since, from PC0, to G0/0/0 we have a (without taking into consideration that these are private IPs of course) range and knowing that HDLC only works on data-link layer, how would PC0 would be able to ping PC1? (Also knowing that from PC1 to G0/0/0 we would have a network

I get that configuring serials on the same net mask via network between both physical links they would manage to ping each other, but what about the rest of the devices? (Again, regardeless that the PC1 is a host... it could be a server, or anything else).

Many thanks for any input!

  • PC0 & PC1 can talk to each other because of ROUTING.
    – Ricky
    Commented Oct 12, 2021 at 19:49
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Commented Nov 20, 2022 at 0:12

2 Answers 2


In that diagram, HDLC is only used on the serial link between the ISRs. Everything else is Ethernet. The WAN link is likely a leased line that may be dozens or hundreds of kilometers in length.

Ping works on the network layer - here that's IPv4. On that layer, PC0 sends an IP packet with an ICMP echo request to its gateway, Router0. Router0 queries its routing table and forwards the packet to Router1 which forwards to PC1.

If you look at the data link layer, things are a bit more complicated. PC0 encapsulates its IP packet in an Ethernet frame, addresses that to Router0's MAC and transmits with 100 Mbit/s. Switch0 forwards by MAC address to Router0 with 1 Gbit/s. Router0 extracts the IP packet.

Router0 uses a serial interface with HDLC towards Router1, so it encapsulates the packet in an HDLC frame and forwards. Router1 extracts the IP packet. Encapsulates it in Ethernet addressed to PC1's MAC and forwards. ...

The end nodes only know what they use with their network connections. They have no idea which physical and data link layer protocols are used in between - whether those use copper wires, fiber, radio, laser, microwaves, avian carriers, ... The IP packet is transparently forwarded across all those links.

(And if you want to look even deeper, at the physical layer where the bits are actually moved, things can get way more complicated...)


Router0 will need a route to, and router1 will need a route to

These routes can be statically (i.e. manually) configured, or the routers can run a routing protocol (e.g. OSPF) to exchange routes.

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