• Today the Internet is on IPv4, IPv4 was good but now due to it Internet has to face some problem, the main and the major problem is growing shortage of IPv4 addresses. There are new machines added on Internet daily and IPv4 is no longer going to support all of these machines.

• This limitation force the organisation to use NAT (Network Address Translation) a protocol to map multiple private addresses into single public address. NATs does not support standards-based network-layer security and also creates complicated barriers to VoIP, and other services.

• The routing tables on the backbone of Internet become larger.

• Source and destination addresses are 32 bits (4 bytes) in length, means 2^32 which amounts to around 4.3 billion addresses.
• Security (IPsec) support is optional.
• Header includes a checksum.
• Require manual configuration or DHCP 
• Send traffic all the node on a subnet (broadcast).
• Sending host and router both has to fragment the packets

IPv6:

• Source and destination addresses are 128 bits (16 bytes) in length, means 2^128 which is 3.4*10^8 (340 undecillion) addresses.
• Security (IPsec) support is required.
• Header does not includes a checksum.
• Do not require manual or DHCP.
• Link-local scope all-nodes multicast address
• Sending host has to fragment the packets.

• OMG! World IPv6 Day Tester 
• Test Your IPv6