IPv6 addressing explained for Network+

Try 3 questions on this

Pick an answer to see why each option is right or wrong.

1. A company has received a /48 IPv6 allocation and is carving out one prefix for each office LAN so hosts there can autoconfigure their addresses. Which of the following prefix lengths should each LAN receive?

   Right. In the game that’s Renown for your Networking Concepts building.

   Not that one: A is right. In the game your Networking Concepts building would lose a little Renown, and this question would come back later.

   The answerA./64RFC 7421 explains that the interface identifier is 64 bits for all unicast addresses and all media supporting SLAAC, which fixes the subnet prefix at 64 bits regardless of the aggregate prefix assigned to the site.Your answerB./48The /48 is the site's whole allocation. RFC 7421 notes a /48 gives 16 bits for hierarchical allocation, so it is divided into many LAN prefixes rather than given to one.Your answerC./56RFC 7421 mentions /56 as an aggregate prefix a site may be assigned. It is a site-level size, not the length of one LAN's subnet.Your answerD./127RFC 7421 mentions /127 for router-to-router links. With only one bit left there is no room for hosts to autoconfigure addresses.

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   Each LAN therefore gets a /64 carved from the /48.

   Written from www.rfc-editor.org/rfc/rfc7421.html

2. Two companies that address their internal servers privately will join their networks over a VPN and may later merge. Which of the following address types is least likely to force either company to renumber?

   Right. In the game that’s Renown for your Networking Concepts building.

   Not that one: D is right. In the game your Networking Concepts building would lose a little Renown, and this question would come back later.

   Your answerA.RFC1918RFC1918 space is private and not routed on the Internet, which fits the internal use. But it is a small set of fixed blocks every organization reuses, so two companies' plans can overlap; RFC 1918 section 4 itself warns that merging private internets may require renumbering.Your answerB.Link-localA link-local address is valid only on the segment where it was formed and is never forwarded by a router, so it cannot address servers reached across a VPN between two sites.Your answerC.APIPAAPIPA addresses are what an IPv4 host gives itself when no DHCP server answers. They are confined to the local segment and are not a plan for addressing servers that must be reached from another company.The answerD.Unique localRFC 4193 unique local IPv6 addresses carry a pseudo-randomly generated Global ID, so each site's prefix is globally unique with high probability.

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   The RFC lists, as a design goal, that sites can be combined or privately interconnected without address conflicts or renumbering. Both companies can keep their prefixes through a VPN link or a merger.

   Written from www.rfc-editor.org/rfc/rfc4193.html

3. A company wants its IPv6 hosts to receive addresses automatically, but its administrators must decide the specific address each host ends up with. Which of the following should provide the addresses?

   Right. In the game that’s Renown for your Networking Concepts building.

   Not that one: B is right. In the game your Networking Concepts building would lose a little Renown, and this question would come back later.

   Your answerA.SLAACSLAAC is automatic, but each host combines the router-advertised prefix with an interface identifier it generates itself. RFC 4862 reserves it for sites not particularly concerned with the exact addresses hosts use.The answerB.DHCPv6RFC 4862 says the stateless approach is used when a site is not particularly concerned with the exact addresses hosts use, while DHCPv6 is used when a site requires tighter control over exact address assignments.Your answerC.APIPAAPIPA is the IPv4 link-local range a host assigns itself when no DHCP server answers. It is not an IPv6 mechanism and gives the administrators no say in the address.Your answerD.Static assignmentTyping each address into each host gives full control, but the requirement is that hosts receive addresses automatically, which static assignment does not do.

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   Both are automatic; only DHCPv6 lets the administrators decide the specific address.

   Written from www.rfc-editor.org/rfc/rfc4862.html