My confusion with IPv6 SLAAC is that the router provides the network prefix, and the host generates its own part using EUI-64, which means it is always going to be a 64-bit address.

Now, SLAAC is part of Router Advertisement and this is the only way to get "Default Gateway" for IPv6 addresses, as I have read that DHCPv6 doesn't provide gateway information. Only Router Advertisements give Gateway information to hosts.

Now, the problem is, if I want smaller subnets in my organization with /96 or /112 or /116 addresses, how can I address this issue, if I want the addresses to be provided dynamically? Because, if I use SLAAC, then I need to keep the host portion as /64 and if I use DHCPv6, I will not get gateway information for my hosts.

Thanks in advance for your help.

  • 5
    You're always supposed to use /64 subnets. If you have more than one network, then you need a larger IPv6 block. – Michael Hampton Feb 22 '16 at 10:01

There is no such thing as /96 or /112 or the like, and you better don't even try to "make" such subnets. Apart from some special cases a subnet in IPv6 is always /64. Several mechanisms rely on that, among which is SLAAC. If you try to invent smaller subnets, these mechanisms will sooner or later collide with your scheme.

In an enterprise environment you are usually assigned a /48 or larger, so there should be no lack of subnets. To think that a /64 is a big waste is tempting but entirely IPv4-ish. The seemingly ludicrous size of subnets was chosen to allow for unmanaged address generation mechanisms (such as SLAAC) that pick an interface identifier randomly (in a suitable sense) with acceptably low collision probability.

N.B. Your statement

I have read that DHCPv6 doesn't provide gateway information.

is not true. It is possible to configure the network such that all relevant information about network configuration is distributed by DHCPv6.

| improve this answer | |
  • Hi. Thanks for the reply, but I tried searching about Gateway configuration on DHCPv6, but not getting it. Being a small organization, a /64 subnet has been provided by the ISP. That's why, I was looking to further break up that subnet into more subnets, as per the internal requirement of the organization. It will be of immense help, if you can guide on "gateway in DHCPv6", meanwhile I will ask my ISP to provide /48 subnet. – Aashish K Goyal Feb 23 '16 at 19:14
  • If you only get a single /64, then your ISP is violating best practice. If you run a commercial site, then they are supposed to give you a /48. If they don't, then complain loudly. ~ I am not familiar with the gory details of DHCPv6 but in stateful mode it does the same as DHCP for IPv4. – countermode Feb 24 '16 at 8:12
  • @countermode DHCPv6 in fact does not distribute gateway information. In IPv6, gateway information is distributed through router advertisement, regardless of whether you use DHCPv6 or SLAAC. The only exception is if you statically configure IP addresses. – Kevin Keane Apr 14 at 2:35

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