I'm using an arduino UNO and an Ethernet Shield to create a web server to response HTTP requests.
The requests are sent by ajax XMLHttpRequest.
It's working fine with static ip address.
But a want the arduindo to get a DHCP ip, so I can use it in any local network (with DHCP).
I want to discover the ip of the arduino connected on the local network.
So I can use it as url to send HTTP requests.
OK, thanks for answering my questions above. That helped layout the network structure and the problem you're trying to solve.
Two computers: 1 web server whose address is dynamic (DHCP) and 1 web client running AJAX and HTML. How can browser find DHCP server?
This is a classic problem solved many different ways throughout the history of computer networks. I've suggested some options below.
tl;dr - Use Option 6 if you don't control your router, Option 5 if you do.
I'm assuming you know your browser machine's IP address (for example, 192.168.1.17). In this case, run through all of the address from 192.168.1.0 through 192.168.1.254 (not .255 and skip you browser machine's IP) testing for a connection to port 80. This will find every web server on your subnet, so be aware you will need a way to recognize your arduino responded to the web request in case some other web server is also listening on the network. It will also take some time to set up, test and wait for timeouts on most (252) of the addresses which don't have web servers. You will eventually find it.
I'm not a fan of this one, but it gets the job done. Warning: if you don't "own" the network, someone may be angry with you for scanning their machines. A company, school or other institution may have policies about not scanning networks.
This one is great, but requires a browser plug-in and some fancy coding. UDP allows one to broadcast a message to your subnet (try
ping 255.255.255.255 at a command line and watch the machines echo back their IPs). If the arduino is set to listen for UDP packets on a particular port, it can echo back to the sender of the UDP packet and let that user know it's present. This is how DNS, DHCP, ping and Apple's Bonjour work. Many IP based systems advertise services by responding to UDP requests. Clients need not know the address of the servers on the subnet, they discover them through broadcast messages on well-known UDP ports.
I don't like any of these modifications for you. Browser, Arduino (Apple, Windows or Linux), just because it adds more moving parts and you're counting on the browser to "know" how to find the service.
This isn't as bad as it sounds, but I'm not sure the Arduino could handle it. Find a very small DNS server written for the Arduino and have it respond to DNS requests. On the browser, look for a well-known machine name (e.g. "my-arduino.lan"). This essentially finesses the UDP problem above by making the Arduino the UDP server (handles DNS requests) and the browser already has name resolution s/w (like every machine on the planet).
I didn't search for Arduino DNS code, it might be too large for the Arduino and writing it may be a real PITA (pain in the butt).
In this model, you can attach a LCD to the arduino and have it present its IP address on the display. Alternatively, the arduino can send a message (via TCP) to a well known server on the network (internet or otherwise) reporting its IP address there. Your browser can query that server, pick up the IP address and then contact the Arduino directly. This introduces a 3rd machine and acts like your own hacked form of DDNS (look it up, if you're not familiar with it).
If you own the router, you can modify the router to assign a specific IP address to the Arduino, even with DHCP. This is your best bet. Here, you control the network, can allow the Arduino to come up in DHCP while still fixing its IP address. You'll have to go through your router API (web or CLI) and figure out how to do it, it's a bit hard directing you as there are thousands of types of commercial and SOHO routers.
If you don't control the router...
This option is really your best. Give up on DHCP and just set the static IP of the arduino. Just make sure the IP address you pick doesn't conflict with any other servers on the subnet. That shouldn't be difficult.
I hope this helps.