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Autonomous Configuration on Legacy Networks Disclosure Number: IPCOM000012038D
Original Publication Date: 2003-Apr-03
Included in the Prior Art Database: 2003-Apr-03
Document File: 2 page(s) / 50K

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Address configuration on legacy networks (e.g. IP) is done using either manually-configured static addresses or with a protocol like DHCP. A better solution is to create an autonomous address configuration mechanism. The above solution uses the existing Address Resolution Protocol (ARP) to solve this problem.

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Autonomous Configuration on Legacy Networks

The subnet router is configured by the system administrator with an IP address and netmask . This is the only configuration required by the autonomous mechanism. Every other host that is attached to the network will compute its IP address automatically as follows:

When a host brings up a network interface, it MUST send an ARP request addressed to the IP broadcast address for the network. Every other host on the network that receives this request MUST respond with an ARP response addressed to the requesting node and including its own IP address. The requesting node waits for a predefined amount of time for these ARP responses. Once it receives the responses, it uses the IP address information in each of them to estimate the netmask for the network (explained further below). Using this estimated netmask, the host then calculates free (unused) IP addresses that fall within the subnet. It picks the first of these free addresses for itself.

It is possible that 2 hosts that are both initializing network interfaces might simultaneously pick the same IP address from the calculated set of free addresses. In order to resolve this conflict, each host that picks an IP address MUST send a Gratuitous ARP packet including this address. If 2 hosts happened to pick the same address, then the host with the lower MAC (network hardware) address loses and must pick a new address from the computed list.

Netmask Estimation: The hosts always estimate the smallest possible netmask for the current number of hosts on the subnet. While new hosts are being added to the network, some hosts may estimate the netmask wrongly. For example, when there are just 4 hosts on the network, a netmask of 2 bits is sufficient. When a 5th host joins the network, all 5 hosts must now readjust their netmasks to...