8Understanding IPv6, Second Edition
Comparison of IPv4 and IPv6
Table 1-1 highlights some of the key differences between IPv4 and IPv6.
Table 1-1 Differences Between IPv4 and IPv6
IPv4 |
IPv6 |
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Source and destination addresses are 32 bits (4 bytes) in length.
Sourceanddestinationaddressesare128bits(16bytes) in length. For more information, see Chapter 3, “IPv6 Addressing.”
IPsec header support is optional.
IPsec header support is required. For more information, see Chapter 4, “The IPv6 Header.”
Noidentification of packet flow for prioritized delivery handling by routers is present within the IPv4 header.
Packet flow identification for prioritized delivery handling by routers is present within the IPv6 header using the Flow Label field. For more information, see Chapter 4.
Fragmentation is performed by the sending host and at routers, slowing router performance.
Fragmentation is performed only by the sending host. For more information, see Chapter 4.
Has no link-layer packet-size requirements, |
Link layer must support a 1280-byte packet and be |
and must be able to reassemble a 576-byte |
able to reassemble a 1500-byte packet. For more |
packet. |
information, see Chapter 4. |
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Header includes a checksum. |
Header does not include a checksum. For more |
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information, see Chapter 4. |
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Header includes options. |
All optional data is moved to IPv6 extension |
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headers. For more information, see Chapter 4. |
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ARP uses broadcast ARP Request frames to |
ARP Request frames are replaced with multicast |
resolve an IPv4 address to a link-layer address. Neighbor Solicitation messages. For more |
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information, see Chapter 6, “Neighbor Discovery.” |
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Internet Group Management Protocol (IGMP) |
IGMP is replaced with Multicast Listener Discovery |
is used to manage local subnet group mem- |
(MLD) messages. For more information, see |
bership. |
Chapter 7, “Multicast Listener Discovery.” |
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ICMP Router Discovery is used to determine |
ICMPv4 Router Discovery is replaced with ICMPv6 |
the IPv4 address of the best default gateway |
Router Solicitation and Router Advertisement |
and is optional. |
messages, and it is required. For more information, |
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see Chapter 6. |
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Broadcastaddressesareusedtosendtrafficto |
There are no IPv6 broadcast addresses. Instead, a |
all nodes on a subnet. |
link-local scope all-nodes multicast address is used. |
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For more information, see “Multicast IPv6 |
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Addresses” in Chapter 3. |
Must be configured either manually or through DHCP for IPv4.
Does not require manual configuration or DHCP for IPv6. For more information, see Chapter 8, “Address Autoconfiguration.”
Uses host address (A) resource records in the |
Uses AAAA records in the DNS to map host names |
Domain Name System (DNS) to map host |
to IPv6 addresses. For more information, see |
names to IPv4 addresses. |
Chapter 9, “IPv6 and Name Resolution.” |
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Uses pointer (PTR) resource records in the IN- |
Uses pointer (PTR) resource records in the IP6.ARPA |
ADDR.ARPA DNS domain to map IPv4 |
DNS domain to map IPv6 addresses to host names. |
addresses to host names. |
For more information, see Chapter 9. |
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Chapter 1 Introduction to IPv6 |
9 |
IPv6 Terminology
The following list of common terms for network elements and concepts provides a foundation for subsequent chapters. Figure 1-3 shows an IPv6 network.
Neighbors
Host
Host
Router
Layer 2 switch
Link or Subnet
Additional
subnets
Network
Figure 1-3 Elements of an IPv6 network
IPv6 common terms and concepts are defined as follows:
■Node Any device that runs an implementation of IPv6. This includes routers and hosts.
■Router A node that can forward IPv6 packets not explicitly addressed to itself. On an IPv6 network, a router also typically advertises its presence and host configuration information.
■Host A node that cannot forward IPv6 packets not explicitly addressed to itself (a nonrouter). A host is typically the source and a destination of IPv6 traffic, and it silently discards traffic received that is not explicitly addressed to itself.
■Upper-layer protocol A protocol above IPv6 that uses IPv6 as its transport. Examples include Internet layer protocols such as ICMPv6 and Transport layer protocols such as TCP and UDP (but not Application layer protocols such as FTP and DNS, which use TCP and UDP as their transport).
■Link The set of network interfaces that are bounded by routers and that use the same 64-bit IPv6 unicast address prefix. Other terms for “link” are subnet and network segment. Many link-layer technologies are already defined for IPv6, including typical LAN technologies (such as Ethernet and Institute of Electrical and Electronics Engineers [IEEE] 802.11 wireless) and wide area network (WAN) technologies (such as the Point-to-Point
10 Understanding IPv6, Second Edition
Protocol [PPP] and Frame Relay). Additionally, IPv6 packets can be sent over logical links representing an IPv4 or IPv6 network, by encapsulating the IPv6 packet within an IPv4 or IPv6 header. For more information about LAN and WAN media support for IPv6, see Appendix A, “Link-Layer Support for IPv6.”
■Network Two or more subnets connected by routers. Another term for network is internetwork.
■Neighbors Nodes connected to the same link. Neighbors in IPv6 have special significance because of IPv6 Neighbor Discovery, which has facilities to resolve neighbor linklayer addresses and detect and monitor neighbor reachability.
■Interface The representation of a physical or logical attachment of a node to a link. An example of a physical interface is a network adapter. An example of a logical interface is a “tunnel” interface that is used to send IPv6 packets across an IPv4 network by encapsulating the IPv6 packet inside an IPv4 header.
■Address An identifier that can be used as the source or destination of IPv6 packets that is assigned at the IPv6 layer to an interface or set of interfaces.
■Packet The protocol data unit (PDU) that exists at the IPv6 layer and is composed of an IPv6 header and payload.
■Link MTU The maximum transmission unit (MTU)—the number of bytes in the largest IPv6 packet—that can be sent on a link. Because the maximum frame size includes the link-layer medium headers and trailers, the link MTU is not the same as the maximum frame size of the link. The link MTU is the same as the maximum payload size of the link-layer technology. For example, for Ethernet using Ethernet II encapsulation, the maximum Ethernet frame payload size is 1500 bytes. Therefore, the link MTU is 1500. For a link with multiple link-layer technologies (for example, a bridged link), the link MTU is the smallest link MTU of all the link-layer technologies present on the link.
■Path MTU The maximum-sized IPv6 packet that can be sent without performing host fragmentation between a source and destination over a path in an IPv6 network. The path MTU is typically the smallest link MTU of all the links in the path.
Figure 1-4 shows an IPv6-capable organization network and its relation to the IPv4 and IPv6 Internets.
A site is an autonomously operating IP-based network that is connected to the IPv6 Internet. Network architects and administrators within the site determine the addressing plan and routing policy for the organization network. An organization can have multiple sites. The actual connection to the IPv6 Internet can be either of the following types:
■Direct The connection to the IPv6 Internet uses a wide area network link (such as Frame Relay or T-Carrier) and connects to an IPv6-capable ISP (shown in Figure 1-4).
■Tunneled The connection to the IPv6 Internet uses an IPv6 over IPv4 tunnel and connects to an IPv6 tunneling router.