use should have a 64- bit mask. ARIN (American Registry for Internet Numbers) will allocate IPv6 networks as follows, according to http://www.arin.net/policy/nrpm.html#six54 End-users are assigned an end site assignment from their LIR or ISP. The exact size of the assignment is a local decision for the LIR or ISP to make, using a minimum value of a /64 (when only one subnet is anticipated for the end site) up to the normal maximum of /48, except in cases of extra large end sites where a larger assignment can be justified. Addressing The following guidelines may be useful (but they are only guidelines): /64 when it is known that one and only one subnet is needed /56 for small sites, those expected to need only a few subnets over the next 5 years. /48 for larger sites While it is technically possible to use a VLSM-type paradigm with IPv6, you shouldn't. Instead, use /64 everywhere. Besides, even if you don't, your link-local address on that interface will be /64 anyway. For reference, a /48 would give you 2 X 1016 (20,000,000,000,000,000) /64 subnets. I think you can spare a couple for your serial links. Imagine-every IPv6 subnet on your network will have a /64 prefix. No more trying to figure out how many hosts exist within a subnet. Each one has 18 quintillion. In other words, for all practical purposes, it no longer matters. The IETF recommends that all IPv6 networks be /64, even for point-to-point links. Further, it recommends that every home network be allocated a /48. You can read for yourself in RFC 3177. Here's a snippet: In particular, we recommend: - Home network subscribers, connecting through on-demand or always-on connec- tions should receive a /48. Small and large enterprises should receive a /48. Very large subscribers could receive a /47 or slightly shorter prefix, or multiple/48's. Mobile networks, such as vehicles or mobile phones with an additional network interface (such as bluetooth or 802.11b) should receive a static /64 prefix to allow the connection of multiple devices through one subnet. - 