Briefly explain how to tunnel through the IPv4 cloud.

What will be an ideal response?

To send a packet to a distant IPv6 network through intervening IPv4 routers, the IPv6 packet is formed normally and sent to a router capable of encapsulating it in an IPv4 packet. The routers on both ends of the tunnels must be dual-stack routers, capable of understanding both IPv4 and IPv6. When the interim IPv4 packet reaches the distant IPv6 dual-stack router, that router strips off the IPv4 packet and routes the packet on its local IPv6 network normally. IPv4-compatible IPv6 addresses were designed expressly for this kind of situation, where IPv6 routers need to understand IPv4.
An alternate scheme is the 6to4 tunneling method, as specified in RFC 3056. For IPv4 nodes or networks with public IPv4 addresses, a legitimate IPv6 address is formed by adding the 32 bits of the IPv4 address to the network prefix 2002::/16. This creates an address of the following form: 2002::IPv4address:SLA:Interface_ID. The interface ID is the normal 48 bits, plus the added 16 bits 0xFFFE, and the seventh bit of the first byte inverted, to create a 64-bit EUI-64 formatted IPv6 address (see RFC 4291). The site-level address is the normal 16 bits, and the combination of the 2002::/16 prefix and the 32 bits of the IPv4 address makes up the first 48 bits of the new address.