Your IP address is:

An Internet Protocol (IP) address is a numerical label that is assigned to devices participating in a computer network that uses the Internet Protocol for communication between its nodes. An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."

The designers of TCP/IP defined an IP address as a 32-bit number and this system, known as Internet Protocol Version 4 or IPv4, is still in use today. However, due to the enormous growth of the Internet and the resulting depletion of available addresses, a new addressing system (IPv6), using 128 bits for the address, was developed in 1995 and last standardized by RFC 2460 in 1998. Although IP addresses are stored as binary numbers, they are usually displayed in human-readable notations, such as (for IPv4), and 2001:db8:0:1234:0:567:1:1 (for IPv6).

The Internet Protocol also routes data packets between networks; IP addresses specify the locations of the source and destination nodes in the topology of the routing system. For this purpose, some of the bits in an IP address are used to designate a subnetwork. The number of these bits is indicated in CIDR notation, appended to the IP address; e.g.,

As the development of private networks raised the threat of IPv4 address exhaustion, RFC 1918 set aside a group of private address spaces that may be used by anyone on private networks. They are often used with network address translators to connect to the global public Internet.

The Internet Assigned Numbers Authority (IANA), which manages the IP address space allocations globally, cooperates with five Regional Internet Registries (RIRs) to allocate IP address blocks to Local Internet Registries (Internet service providers) and other entities.

Courtesy of Wikipedia under the CC-BY-SA license.

Your IP address is assigned by your ISP (Internet Service Provider). In most cases, you can't easily change this address or choose which IP address you are using.

If your IP is dynamically assigned, then you may be able to disconnect then reconnect to the internet to get a new IP address from your service provider. This generally only works for people with dial-up internet, but may work with DSL and cable internet connections.

If you are wanting to protect your privacy on the internet, you can use a service like Tor. Tor protects you by bouncing your communications around a distributed network of relays run by volunteers all around the world: it prevents somebody watching your Internet connection from learning what sites you visit, and it prevents the sites you visit from learning your physical location. Tor works with many of your existing applications, including web browsers, instant messaging clients, remote login, and other applications based on the TCP protocol.

Every email contains headers. The headers contain information about where the email is going and where it has been. When you send an email, it may go through several mail servers before it reaches the recipient. Whenever an email goes through a mail server, an item is added to the headers indicating which server the email has gone through. The headers can sometimes help track down the originating mail server, but they do not contain any personal information such as name or address.

Most spammers use several methods to hide their identities. For example, they add false headers to the email to make it look like the email originated from a different server than the actual sending server. They may also include a false "from" address, making it impossible to send an email to the spammer or to get their email account suspended.

An email message, like a letter sent through snail mail, has an envelope. Like with postal mail, the sender can put anything they want as the "from" name and address. There is no way to verify if the name and address put on the envelope is the real name and address of the sender. With spam, the name and address of the sender is almost always fake. To make it worse, the name and address often belong to a real person that doesn't even know that you are receiving spam that looks like it is from them. In the "real world", you could compare this to a neighbor sending letters with your name and address as the return address. People may get angry that "you" are writing them, but you don't even know the letters exist! Because of this, it is generally inappropriate to reply to spam in any way.