Google it<\/a>.<\/p>\nIn a real point-to-point interface, both ends of the link know the semantics of the link.\u00a0 For Ethernet point-to-point addressing, we can still do this (and my code happily supports this configuration), but obviously both ends have to agree to do so. “Normal” clients won’t know what we’re up to, so we have to do this in such a way that we don’t upset their assumptions.<\/p>\n
So we cheat. And we lie. And worst of all,we do proxy ARP!<\/p>\n
What we do is tell our clients that they are on a \/24 network. Their IP address is, for example, 10.1.2.5\/24, and the gateway is 10.1.2.1. Any time we get a packet for 10.1.2.5, we’ll send it out that interface, doing ARP as normal to resolve the remote host’s MAC address.<\/p>\n
Going the other way, we answer ARP requests for any IP address in 10.1.2.0\/24, except 10.1.2.5, with our own MAC address.\u00a0 That means that if they ARP for 10.1.2.6, we’ll answer the ARP request, which directs that packet to us, where we can use our interior routes to route it correctly.\u00a0 In our world, two “adjacent” IP addresses could be on opposite sides of the network, or it could be on a different VLAN on the same interface.<\/p>\n
The result is one IP address per customer.\u00a0 We “waste” three addresses per 256, the network (.0), gateway (.1) and broadcast (.255), and we have to be a bit careful about what we do with the .1 address — it could appear on every router that is playing with that \/24.\u00a0 But we can give a user a single IP address, and put it anywhere in the network.<\/p>\n
We can actually have multiple IP addresses on the same interface; we do this by having the NetGraph module have a single Ethernet interface but multiple virtual point-to-point interfaces.\u00a0 So if we want to give someone two IP addresses, we can do that as two, not necessarily adjacent, \/32 addresses.\u00a0 We don’t answer ARPs for any of the assigned addresses, but do answer everything else. The module maintains a mapping of point-to-point interface to associated MAC address.\t\t<\/p>\n","protected":false},"excerpt":{"rendered":"
I hate IPv4 link broadcast interface (e.g. Ethernet) addressing semantics.\u00a0 To recap, if I have two boxes on each end of a point-to-point link (say between a gateway and an end host), we address as follows (for example): 10.1.1.0: Network address (reserved) 10.1.1.1: Host 1 (gateway) 10.1.1.2: Host 2 (end host) 10.1.1.3: Broadcast address. That’s […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[8,11,16,20,22,24],"class_list":["post-62","post","type-post","status-publish","format-standard","hentry","category-pushing-packets","tag-arp","tag-broadband","tag-ethernet","tag-ipv4","tag-packets","tag-point-to-point-ethernet"],"_links":{"self":[{"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/62","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=62"}],"version-history":[{"count":0,"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/62\/revisions"}],"wp:attachment":[{"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=62"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=62"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/don.nz.net\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=62"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}