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With the increase of hand held technology like cell phone, laptops and other mobile devices, wireless access for internet is required for all mobile devices. So that internet facility is available for everyone and everywhere. For this facility wireless interfaces are used so that network link changed automatically by the devices when it moves from one link to other. For uninterrupted communication Mobile IP technology is introduced . This is an enhanced standard Internet Protocol introduced by the Internet Engineering Task Force (IETF), designed for making mobile computing a reality and to solve the mobility problem of network node . MIPV4 & MIPV6  are basic protocols supports the IP for mobile devices. 
Next-generation mobile systems support the IP-based services, with the coupling of mobility with QOS. This mobility of devices from one location to another is same model as changing of network node points.MIPV4 and MIPV6 are introduced by small changing in current IPV4 and IPV6 respectively, which support the mobility. Mobile IP is operated in IP layer but application layer IPs remain unchanged. In this technology users are allocated by any IP address even they are disconnected from their own network. In MIPv4, the Mobile network is externally assigned by a new IP address from a foreign router (foreign agent (FA)) in the visited network. Mobile network update its location with its home agent (HA), which forwards the packets, when its moves to a new location, to maintain continuous connectivity. MIPv6  is also designed for supporting mobility and overcoming some problems of MIPv4. By the increase in the number of potential users of such services within business, industry and the private sector will increase the need of next generation version of IP (IPv6).
MIPV4 is used to provide the access to users anywhere in the IPV4 network. IP addresses are 32 bit long in the Mobile IPV4 and denoted in the dotted decimal form (e.g., 129.65.36). Each IP contains the IP header and IP payload. Functionally used in the MIPV4 is the foreign agent routers functionality.
MIPV4 consists of mobile node/mobile host (MN/MH), home agent (HA) that is located in the home network and maintains the activity information on MN, and foreign agent (FA) manages its network when Mobile Node is moving and work as a care-of-address (COA). And corresponding Host is node that transfers the information to the MN , . FA continuously sends the Agent advertisement messages; MN receive these messages if and only the MN is located in the area that is managed by FA. When message is received by the MN, it starts location enrolment process by transferring the registration request message with COA being FA to HA through FA. And the HA updates activity information on MN and sends back the enrolment response and fasten message. When mobile node moving between different networks, HA tries to locate the location by registration demand and answer. If corresponding node (CN) wants to connect with MN, the CN transfer the data message to MN.
Due to standard internet routing, data send from CN and received at the home network of Mobile node, particularly HA. HA checks the fasten list from the data destination address and sends the data to its FA only if MN is not located in the home network. FA also confirms the visitors list from the data destination address which is received from HA. And then transfer the data to MN, if mobile node is located in the foreign network . 
Fundamentals and operation of Mobile IPV4
Since when IPV4 was make there was no any concept of access, anywhere anytime.
Mobile IPV4 was an advance version of IPV4 with support to the access everywhere any time. Mobile IP soles the many problems by giving the IP addresses to the MN on an ordinary basis, and manages the real IP address of MN . Functionality used in MIPV4 is the foreign agent functionality. MIPV4 consists of MN, HA that located in the home network and keeps the activity information on MN and FA manages its network when MN is moving and behaves as a care-of-address for MN. And CO node is the node which is used to transfer information or to connect with MN. 
The three main parts of Mobile IP are Agent discovery, registration and tunneling. Functions performed by these three parts are: 
Agent Discovery: Mobility agent declares their availability on every link to whom they provide the service.
Registration: When MN is not located in the home, it enrolls its care-of-address with HA.
Tunneling: If data, wants to be transferred to the MN when it is not in the home, the home agent transfer the data to the care-of-address.
The operation of MIPV4 is shown in Figure 1. 
Figure 1. Operation of MIPV4
Mobile Ipv6 is based on new features and overcome many problems that is experienced in mobile IPV4.The Main advantage in MIPV6 is that it is based on the new IPV6 protocol. IP addresses are 128 bit long integers, MIPV6 provide the mobility that solve many problems (tunneling, ingress filter).Mobile IPV6 is the new generation of IPV6 that provide the mobility. Unlikely in MIPv4, Functionally is not present in MIPV6 because mobile node has its new IPV6 address when it is in a foreign network by address auto-configuration method. .
In MIPV6, router advertisement message explain the MN's mobility and node's neighbor unreachable detection mechanism. MN makes a reason for router to send advertisement message by requesting if it is necessary. When its mobility is detected after detecting its mobility, the MN obtains COA unlikely in MIPv4. And MN sends the fasten update message to the HA and the communicated CN. The Home Agent and corresponding Node update their combining list and send the recognition message. If a CN wants to impart with MN, the CN sends the data to original MN's IP address, because it does not know the MN's COA .Therefore, data is received at the home network through standard IP routing. The Home Agent confirms its binding list with its destination address and then sends the data to the MN's COA. The MN, when received the data send the combining update message to the CN, After receiving the newly received message CN transferred data to MN's COA . . 
Fundamentals and operation of MIPV6
MIPV4 is not widely used to provide access to user anywhere and has many disadvantages like limited number of IP addresses.
Another main problem is that the devices that required the IP addresses to access the internet is increasing rapidly. To solve this limited IP addresses problem, IETF has introduced the MIPV6. MIPV6 provide large number of IP addresses to mobile users to access the internet and access everywhere, anytime even when users are moving between different networks . MIPV6 has new version which is designed to overcome the problems in IPV4. MIPV6 is based on the new IPV6 protocol. In MIPV6 IP addresses are 128 bits long, and it provide mobility to all users which solve many problems. Functionally is not present in MIPV6 because mobile node has its new IPV6 address when it is in a foreign network by address auto-configuration method. 
The operation of MIPV6 is shown in Figure 2. 
Figure 2. Operation of MIPV6
COMPARISON BETWEEN MIPV4 & MIPV6
The structure of Mobile IP in IPV6 is advance and has the advantage over the Mobile IPV4 and the convenient time given by IPV6. Mobile IPV6 provides many new functions with Mobile IPV4, But these all are combined in the IPV6 and provides many new betterments. This section explains the major differences between Mobile IPv4 and Mobile IPv6 .
â™¦No need to extend the special routers as "foreign agents", like in Mobile IPv4. Mobile IPV6 can work and provide access in any location without any favor from local router.
â™¦To work for betterment, of course is the basic part of protocol, instead of non-standard enlargement.
â™¦Mobile IPv6 route optimization provide security across path. It is considering that path betterment can b extended on a large scale between all MN and their suitable nodes.
â™¦Support is also collected into Mobile IPv6 for course betterment to work with already exist routers that execute "ingress filtering".
â™¦Any change in position finding process in Mobile IPV6 gives two-directional sanction of a MN's skills to connect with its default router at its present location.
â™¦When MN is not located in the home, most packet are transferred in Mobile IPV6 using an IPV6 routing header instead of IP encapsulation, which minimize the amount of resulting as compare to mobile IPV4.
â™¦Mobile IPv6 is separated from any link layer, as it starts using neighbor Discovery except ARP.
â™¦The usage of IPV6 encapsulation and routing header terminate the management of "tunnel soft state".
â™¦The process to discover the home agent address in Mobile IPv6 sends back a single answer to the MN. But in IPV4 separate answers comes from every home agent. 
ADVANTAGES OF MOBILE IP
Mobile IP is a set of criterion used to accommodate networking to an increasingly mobile surrounding. The advantages of Mobile IP are as follows:
Convenience: Mobile IP's biggest benefit is to support internet connection while the user is traveling between different connections. In past, a device should have to b connected again as it move between different networks, but Mobile IP gives hand-off solution that allow the device to move freely within different networks without losing the coverage.
Scalability: Since Mobile IP built for very wide and costly networks that contain many users, network can be easily pended to include wide area and allow large number of user to be connected. Either it is on cellular networks or particular wireless LANs; a Mobile IP setup can spend by adding new connection points.
Consistency: When a user is connected to the network an IP is assigned to user that identifies the user on the network. And allow a router to be connected to that part of the network, or subnet, to send them their request data. By using Mobile IP user kept same IP address across all of the subnets when they are travelling between the routers on their home subnet intercepts and forward all coming data to them. This provides the user with a seamless and constant mobile connection.
Standardized: Since Mobile IP's beginning, it has been chosen as a standard and many new advance networking features are added in them. Companies, such as Cisco, give Mobile IP solutions with a number of their tools.
Integration: Mobile IP solutions do not require a supplement art network, but rather mix with a current network schema. This thing allow the usage of Mobile IP in any situation easily
Uniqueness: A mobile IP address allows the users to connect the internet by using the unique IP address. This unique address allows the computer to communicate home IP address and also communicate with network protocol.
Portability: IP creation allows the user to access the internet while away from the router. Mobile IP provide a tunnel through which user can access the internet from any location. Mobile IP address allows the user to keep an IP address. This option is very beneficial for the users who are travelling. 
Mobile IP is a protocol, which provides mobility support to wireless Internet users. In this paper we discuss the detailed description of MIPV6 & MIPV4, which elaborates the basic operation, protocol, comparison and benefits of Mobile IP. In the earlier, mobility was not a big issue, when internet protocols (IPV4 & IPV6) was introduced. But with the increase in usage of hand held devices, mobility is the main issue. So to overcome this enhanced IP protocols MIPV4 and MIPV6 are designed. Mobile IP is developed by the IETF.