Cookies:-
Cookies provide a useful means in Web applications to store user-specific information. For example, when a user visits your site, you can use cookies to store user preferences or other information. When the user visits your Web site another time, the application can retrieve the information it stored earlier.
A cookie is a small bit of text that accompanies requests and pages as they go between the Web server and browser. The cookie contains information the Web application can read whenever the user visits the site. Imagine that when users request a page from your site, www.contoso.com, your application sends not just a page, but a cookie containing the date and time. When the user's browser gets the page, the browser also gets the cookie, which it stores in a folder on the user's hard disk. Later, the user requests a page from your site again. When the user enters the URL www.contoso.com, the browser looks on the local hard disk for a cookie associated with the URL. If the cookie exists, the browser sends the cookie to your site along with the page request. Your application can then determine the date and time that the user last visited the site. You might use the information to display a message to the user, check an expiration date, or perform any other useful function.
Cookies are associated with a Web site, not with a specific page, so the browser and server will exchange the www.contoso.com cookie information no matter what page the user requests from your site. As the user visits different sites, each site might send a cookie to the user's browser as well; the browser stores all the cookies separately.
Limitations
§ Most browsers support cookies of up to 4096 bytes. That is plenty of space for storing a few values on the user's computers, but you would not want to try to store a dataset or some other potentially large piece of data in a cookie. More practically, you probably do not want to store a big collection of user information in a cookie.
§ Browsers also impose limitations on how many cookies your site can store on the user's computer. Most browsers allow only 20 cookies per site; if you try to store more, the oldest cookies are discarded.
§ A cookie limitation that you are likelier to run into is that users can set their browser to refuse cookies. You cannot do much to get around this problem except to avoid cookies altogether and use a different mechanism to store user-specific information.
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