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cisco ccna youtube.82 Chapter 1 Network Protocols and Network Device BasicsYes, the :: can only appear once in an IPv6 address. Take the IPv6 address FE00:0000:0000:0001:0000:0000:0000:0000:0056 as an example. According to the first principle above, the IPv6 address can be written as FEOO:O:0:1:0:0:0:0:56. The second principle above applies to groups 2, 3, or 5.7 hexadecimal numbers of this IPv6 address, but not both. Choosing the latter application makes this IPv6 address look more concise, as shown below.As you can see from Figure 1-66, the most obvious difference in the figure would be the format of the IPv6 address compared to the previous example of communication between IPv4 hosts. In addition, there are some less obvious differences. For example, how does an IPv6 host learn the IP address of the default gateway? Do IPv6 hosts still need to rely on ARP to learn the MAC address of a known IP address and perform traffic forwarding within the same subnet?As mentioned at the beginning of this section, only a few protocols that were used for IPv4 need to be modified for IPv6, and some protocols that were used for IPv4 no longer work for IPv6, so some new protocols have to be "tailored" for IPv6. For example, ICMP and DHCP could not be used "as is", so new versions were developed for IPv6: ICMPv6 and DHCPv6. For IPv6, IPv4ARP was functionally replaced by a new protocol called IPv6 Neighbor Discovery Protocol. Protocol developers have modified OSPF, EIGRP, and several other routing protocols to introduce new versions (e.g., OSPFv3, EIGRPv6, and RIPng, etc.) for use with IPv6.1.7.1 IPv6 Packet Headers While IPv6 is committed to providing similar functionality to IPv4, it is a new protocol in its own right and is not an improvement on IPv4. Therefore, RFC 2460 defines a new packet header format specifically for IPv6 packets. The format of an IPv6 packet header is shown in Figure 1-67.Most of the fields in the IPv6 packet header serve the same purpose as the IPv4 packet header.The extended header fields introduced by the /gffl IPv6 packet header are the most significant difference between the two packet headers. With the exception of the packet header's fixedzero 58 field in the packet header, IPv6 packets can also carry multiple extended headers containing Layer 3 protocol information. The extended headers are located between the IPv6 packet header and the net load of the IPv6 packet. The next header field value in the IPv6 packet header is used to indicate the type of extended header that immediately follows the IPv6 packet header. If the IPv6 packet does not contain an extended header, the next header field value is used to indicate the Layer 4 protocol header (for example, the TCP header) that immediately follows the IPv6 packet header. The next header field is used in the1.7 IPv6 Basics 83then the next header field value will indicate the type of extension header that immediately follows the IPv6 packet header. IPv6 packets can carry multiple extension headers, and the extension headers are arranged in a chained fashion. Each extension header contains a next header field that indicates whether other types of extension headers follow immediately after it. The value of the next header field of the last extension header in a chained arrangement indicates the immediately following Layer 4 protocol header (e.g., TCP header).