Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

When an ADSL modem has booted and makes a connection, what information is sent across the wire and what is received? Does this differ if the ADSL modem has been assigned a static IP address or a dynamic IP address?

share|improve this question
up vote 2 down vote accepted

What I've figured out so far:

Most ADSL connections use either PPP over Ethernet over ATM ("PPPoE" mode), or DHCP over Ethernet over ATM ("DHCP" mode).

The former, also called PPPoE, uses the same PPP protocol as used by dial-ups – you authenticate with a username/password and get assigned an address. All packets going to the Internet are tunneled inside this PPP connection, so there's a slight overhead.

In the second case, your router contacts the ISP's DHCP server directly and (probably) authenticates you based on the physical phone line, and further data packets are sent directly. The DHCP protocol is the same as used in your home network.

The process is the same for both static and dynamic addresses – the DHCP/PPP server is simply told to reserve a specific address for you, instead of picking any available one.

PPP, IP, DHCP and everything else normally goes over Ethernet – but in this case, the Ethernet frames are further encapsulated in ATM cells and are sent over a statically-configured ATM "virtual circuit". (In case of PPPoA, there is no Ethernet framing involved – PPP data is sent over ATM directly.)

Finally, everything is converted into electrical signals and sent over the phone line.

* [ATM [Ethernet [IPv4 [DHCP]]] → [ATM [Ethernet [IPv4...]]
* [ATM [Ethernet [PPP handshake]]] → [ATM [Ethernet [PPP [IPv4...]]]]
* [ATM [PPP handshake]] → [ATM [PPP [IPv4...]]] least, that's how it seems to be working here. I may be horribly wrong, for I have never worked at an ISP and just like messing with networks.

share|improve this answer
Thanks. If most ADSL connections use PPPoE modes, how is that different to PPPoA? Also does that mean PPPoE does not utilize DHCP when being an IP address? When connecting to a DHCP server, is your MAC address provided by the modem? – PeanutsMonkey Aug 20 '11 at 6:05
@PeanutsMonkey: 1) PPPoE and PPPoA differ in protocol layering: in PPPoE the PPP connection is established over Ethernet packets sent over an ATM circuit, which can be bridged to your home Ethernet; for example, Windows XP has built-in PPPoE functionality, which I used to use when I had a really shitty ISP-provided modem. On the other hand, PPPoA means the PPP packets are sent over the ATM circuit directly, without any Ethernet framing in between; this can only be done by your modem itself. So PPPoE is somewhat more flexible, but has a 14-byte overhead in the most common scenario. – grawity Aug 20 '11 at 6:35
@PeanutsMonkey: 2) PPP in any form does not use DHCP between client and server. The address configuration is done by the PPP protocol, whether over PPP dial-up, PPTP VPN, or PPPo[AE] DSL. (How the PPP server allocates addresses is unknown. It may be contacting a hidden DHCP server, but that is an implementation detail and nothing the user should be concerned about.) – grawity Aug 20 '11 at 6:37
@PeanutsMonkey: 3) Yes. The DHCP protocol uses standard IPv4, which is further encapsulated into Ethernet frames. Those Ethernet frames contain the MAC addresses of both ends. (Although note that most ADSL routermodems have the ability to change their MAC addresses, so this is not a secure authentication mechanism... some ISPs still use it, though.) – grawity Aug 20 '11 at 6:40
Thanks. So seeing that Windows XP has built-in PPPoE functionality, does this mean I can connect directly to the ISP without the need for a modem? I didn't quite understand what you meant by PPP in any form does not use DHCP between client and server. When you sau that most ADSL router modems have the ability to change their MAC address, does this occur dynamically or is it user driven? – PeanutsMonkey Aug 20 '11 at 7:12

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.