H

handshake

See TCP handshake.

hash code

A unique, mathematical summary of a document that serves to identify the document and its contents.
See message digest.

header

A series of bytes at the beginning of a communication packet that provides information about the packet such as its computer of origin, the intended recipient, packet size, and destination port number. The header of a packet is like the envelope of a traditionally-mailed letter, in that it conveys "return address" and "intended recipient" information but is not the real content of the message.

hexadecimal

A base-16 numbering system (from hexadecem, Latin for 16) particularly important in computer programming, since four bits (each consisting of a one or zero) are succinctly expressed using a single hexadecimal digit. Hexadecimal resembles decimal (base-10) numbering with the digits 0 through 9, but the decimal equivalents of 10 - 16 are represented in hexadecimal by the letters A through F. Example: the decimal number 252 is written in hexadecimal as FC.

hierarchical trust

A method of organizing "trust" within an organization by allowing one Certificate Authority to delegate a portion of its responsibility to a subordinate Certificate Authority. For example, a business might have a master Certificate Authority, which vouches for a Certificate Authority at the company's Los Angeles office, which vouches for a Certificate Authority at the company's Phoenix office. Commonly used in ANSI X.509 certificates.

High Availability

High Availability enables the installation of two devices so that if one fails for any reason, the other takes over immediately. This minimizes data loss while the failed device is replaced or repaired..

HMAC (Hashed Message Authentication Code)

A mechanism for message authentication, using cryptographic one-way hash functions, based upon RFC 2104 and commonly used in VPNs. The end result is that when you receive a data packet, you can know that whoever sent the packet possesses the same secret key that you do. You can combine this with other technologies, such as IKE, to know who sent a given message.

home page

The first page of a multi-page Web site, used as an entrance into the site.

host

A network-connected computer.

host route

A network configuration where a router sits between the network device and an internal host. For the device to be able to send data to the host, it must be informed of the existence of the additional router (and the host behind it).

HTML (HyperText Markup Language)

A simple programming language used to format Web pages, including methods to specify text characteristics, graphic placement, and links. HTML files are written in plain text, then read or interpreted by a Web browser.

HTTP (HyperText Transfer Protocol)

A communications standard designed and used to transfer information and documents between servers or from a server to a client. This standard is what enables your Web browser to fetch pages from the World Wide Web.

HTTPS (Secure HTTP)

A variation of HTTP enabling the secure transmission of data. Generally used in conjunction with Secure Sockets Layer (SSL), which encrypts the HTTP.

hub

A device that serves as a common connection point for multiple devices on a network. There are several different types of hubs, but in general each receives and sends signals to all the devices connected to it.

hyperlink

An object on a Web page such as a graphic or underlined text that represents a link to another location, either on the same Web site or on a different Web site. When a user clicks on a hyperlink, a page or graphic from the linked location appears in the user's Web browser.