Multicast DHCP


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Not the final solution

DHCP alone didn't solve all IT's management issues as offices became more interconnected. Think back to the first networked laser printers with an Ethernet port on the back. To send a document to that printer, you needed to install a driver and tell the driver what the printer's address was before you hit print. DHCP could take care of addressing the printer when it booted up, but that won't help you find the printer when installing the driver. Plus, with DHCP being dynamic, what if the address changed? It would be a lot easier for everyone if a user could just hit print and have the computer tell the user what printers it could find.

That's where Multicast DHCP comes in. Apple calls it Bonjour. Microsoft calls it UPnP (Universal Plug and Play). Collectively, it's known as Zeroconf. The end result is that you can plug in a Vista desktop, a Macbook and a Linux Netbook on the same switch and see each other's shared files without a lengthy setup process, and all three of those machines can see the printers on the network without fuss. It just works.

The Logitek Jetcast Mini uses Multicast DNS and Multicast DHCP to identify network resources.

The Logitek Jetcast Mini uses Multicast DNS and Multicast DHCP to identify network resources.

If a modern home or office network can virtually set itself up, why can't an IP audio network? Using the same principles of Multicast DHCP, it can.

Three technologies come together to make everything happen: link-local addressing, Multicast DNS, and DNS Service Discovery.

Link-local addressing, also known as AutoIP, is how a device assigns itself an IP address when there isn't a DHCP server around to hand it an address when it starts up. This is needed for two scenarios: very small LANs where communication is desired without the overhead of running a DHCP server, and a safety net for devices to continue to communicate while the DHCP server is down.

When a device starts and cannot contact a DHCP server, it randomly pulls an address from the link-local pool, which is 165.254.1.0 to 165.254.254.255. Because there is no central authority keeping track of the addresses used, the machine must test to see if the address it chose is in use. If it passes the test, it announces to the world what it is and what its address is; if it fails, it picks another address at random and starts the process again. If a DHCP server joins the network, machines will then ask it for a new address and let it keep order from that point forward.

Machines that receive link-local addresses can only communicate with each other on the local network; they cannot communicate with other subnets or the Internet. This is because its link-local address is only guaranteed to be unique on the local network.

Of course, in a radio station, users prefer to look for audio with sensible names such as “ISDN 1” and “Air Monitor” instead of IP addresses such as 192.168.3.200. Plus, with addresses changing dynamically, it's critical that ISDN 1 is always ISDN 1 and not CD 1 after addresses were reassigned. Multicast DNS manages this for us because users are looking for services instead of IP addresses.



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