Dear Professor Phil,
What happens when a device connects to an IP network?
Simon, West Chester, PA
Simon, you’ve asked a question about something that all of initiate on our computers every day but rarely contemplate exactly what it requires. As soon as there is a physical connection to a network switch, your network card negotiates a speed with the switch. When that succeeds, a link light usually appears on the side of the physical connector where you plug in your cable.
Your pc must know several critical values to function with other network devices. These ordinarily include an IP address, subnet mask, local router, and DNS (domain name services) server. If the device associates automatically with a server, such as entering a Windows domain or connecting to a VoIP call server, it may also need the servers’ IP addresses.
The address, mask, router, and DNS server can be configured in a Windows screen. Or, on that screen you can tell your software to obtain the values automatically by using DHCP (dynamic host configuration protocol). If you choose automatically, your software will broadcast a request and a server will supply the values that your software requires.
When the first network request is ready to be sent, for example to google.com, the following sequence occurs:
1) Your browser knows the name google.com but not the corresponding address. So, it creates a DNS query to be sent to the DNS server. If the server is on the local network, your software sends an ARP (address resolution protocol) broadcast asking for the DNS server’s hardware address.
2) The DNS server responds with its hardware address and the request can be forwarded to the server. Your pc software places the hardware address in a cache (memory buffer) for use with future requests.
3) The request to connect to google. com can be created and sent. However, if your software doesn’t have the router’s address, it may then need to send an ARP broadcast to get it.
Phil Hippensteel is a professor of Information Systems at Penn State Harrisburg. If you have a question for Professor Phil, email it to AVTintern@nbmedia.com.