Hot answers tagged

332

The MAC address might be unique, but there's nothing special about the number that would indicate where it is. MAC 00-00-00-00-00-00 might be on the other side of the planet from 00-00-00-00-00-01. IP is an arbitrary numbering scheme imposed in a hierarchical fashion on a group of computers to logically distinguish them as a group (that's what a subnet is). ...


70

Because the routing tables would become impossibly large. IP addresses are allocated hierarchically, so a router can group routes by address prefixes. The number of autonomous systems present on the net now is reasonable enough to fit in today's hardware. On the other hand, the distribition of MAC addresses across the network is random and completely ...


66

No, they are completely incorrect. In fact, this is a valid IP address: 192.168.24.0 As is 167.23.0.1. Separation of the IP address into dotted segments is a purely human convenience for display. It's a lot easier to remember 192.168.1.42 than 3232235818. What matters to computers is the separation (netmask). It's not valid to have an host address with ...


66

Disclaimer: No offense, but this is a really bad idea. I do not recommend that anyone do this in real life. But if you give a bored IT guy a lab, funny things will happen! For this experiment, I used a Microsoft DNS server running on Server 2012 R2. Because of the complications of hosting a DNS zone in Active Directory, I created a new primary zone named ...


36

It's also reserved for loopback, so no, it's not widely used for anything. In practice, 127.0.0.1 is usually used as "the" loopback address, but the rest of the block should loopback as well, meaning it's just generally not used for anything. (Though, for example, larger Cisco switches will use 127.0.0.xx IPs to listen for attached cards and modules, so at ...


35

The world doesn't run exclusively on ethernet(at least historically). The IP layer is independant of the hardware layer beneath it. PPP nodes don't have Mac addresses. Neither do arcnet, token ring, fddi, hppi. Those other standards may not be as relevent today, but ethernet may be replaced with other technologies in the future and it would be transparent ...


31

Both of course. IPv4 will stay a long time, and it's way past time to start with IPv6.


29

Further to the hierarchical routing of IP, having them separate from MAC addresses allows you to change your network card or whole computer while retaining the same IP address (and thus logical network topology). This abstraction allows for much more flexible and maintainable networking.


28

IPv4 and IPv6 are separate protocols that don't talk to each other. You'll have to support both protocols for now. Getting IPv4 addresses is getting more difficult and expensive, but you'll have to make your service available over it because not all users will have IPv6. On the other side there will be users who don't have full IPv4 anymore. They might have ...


23

IPv6/IPv4 preference is determined by the initiator of a connection, i.e. the web browser. The address selection rules are defined in RFC 6724. While these can be overridden, it is only by the user reconfiguring their operating system. The only way you can force someone to use IPv4 is to not offer IPv6 at all. Obviously this is not a practical solution even ...


22

Take a look at the OSI model: http://en.wikipedia.org/wiki/OSI_model This explains why it doesn't make sense to make routing, a layer 3 concept, decisions based on a physical, layer 2, mechanism. Modern networking is broken into many different layers to accomplish your end to end communication. Your network card (what is addressed by the mac address ...


21

Let me show my working here... You need a minimal number of CIDR blocks to cover: 0.0.0.0-9.255.255.255 11.0.0.0-172.15.255.255 172.32.0.0-192.167.255.255 192.169.0.0-223.255.255.255 To turn these ranges into minimal CIDR blocks, you can just use netmask (the swiss army knife of addressing), like so: $ netmask -c 0.0.0.0:9.255.255.255 0.0.0.0/5 ...


21

Short answer: it will work, technically, but you will have lots of undeliverable mail. Long answer: Take your SMTP logs. Sed out all the domain names you send mail to. Check if they have IPv6 DNS and MX. Once you get 100% (you won't, not anytime this decade), then you can try if the IPv6 IPs actually work. I don't have any interesting production logs at ...


20

If you're a network operator, the first step should be IPv6-enabling your network. Talk to your upstream provider(s) about adding IPv6 to your existing service arrangement. If they're not ready or it looks to be too much of a hassle, consider using a tunnel broker. I've had good experience with Hurricane Electric and they will allocate you a /48 and peer ...


20

Maxmind is a good service, though occasionally there can be errors, since we're now in the time period where IPv4 blocks are scarce, and are being traded and resold on a gray market. If you do find an actual error you can report it to them, though this doesn't appear to be an error. This is basically how I confirm the location of an IP address: First, I'll ...


20

The answer depends on your success criteria. But most likely will be no. If you are running a business where any undelivered mail means a measurable cost. Then the answer is no, IPv6-only is not viable yet. There are many providers including some large providers who are still running IPv4-only. The largest provider I know of with dual stack support is ...


19

Simpler IPs a normal human might be able to remember. Less programming, less memory, etc. Familiarity Thousands (millions?) of Network and System Administrators already know it. Existing infrastructure supports it at almost every level already.


18

To answer the question as it was stated ("how many IPs can a single DNS A record hold?") the answer is very simple: a single A record holds exactly one address. There can however be multiple A records for the same name.


17

The two are unrelated. The decisions around how to structure IPV6 are myriad. There's a lot of info at the IPv6 Wikipedia article. Basically, the 128-bit address space of IPv6 gives us such a massive address space that we are unlikely to ever use all of it (2^128 addresses, or 3.4*10^38). The larger address space also allows for a better hierarchical ...


17

There are two things you need: First you need an ISP that will act as the sponsoring LIR for you. Their role is just book keeping and maintaining the contractual chain between you and RIPE NCC. Then you'll need an ISP that will route your addresses and announce them to the rest of the world using BGP. Those two functions can be provided by a single ISP ...


16

If you host multiple vhost domains with a single Nginx instance, you can't use the single combined listen directive listen [::]:80 ipv6only=off; for each of them. Nginx has a weird quirk where you can only specify the ipv6only parameter once for each port, or it will fail to start. That means you can't specify it for each vhost domain server block. As ...


15

Unless I'm misunderstanding, your testers are dead wrong. Valid IP addresses can certainly have a 0 in them.


13

Such preferences can be expressed using SRV records. Unfortunately those are not supported for HTTP. So you are left with a situation where the client alone is making the choice between IPv4 and IPv6. Many clients use the roundtrip time of SYN + SYN-ACK to decide which of the two to use. So by slowing down the sending of a SYN-ACK packet on IPv6, you can ...


12

Well after a little chat on the IRC the general opinion is that the linux kernel shares some code between ipv4 and ipv6, and that may make completely disabling ipv4 entirely impossible. You can try to compile the kernel without the ipv4 parts but ipv6 may not compile in this case (but nothing keeps you from trying!). You can remove the ipv4 addresses from ...


11

In general: No, it doesn't matter if there is a 0 in the address or not. However, there is a grain of truth in what your testers are saying. In some cases old or broken network equipment will not work correctly on addresses with 0 in the last octests. This is due to the old classfull routing rules. In Classfull routing, you can tell the netmask from the ...


11

Everyone is still using IPv4 addresses, for any common use on the Internet. The person in Japan will have contracted with an ISP there, and would have been assigned an IPv4 address from the pool of available addresses for that ISP. The address blocks may have been fully assigned to ISPs, but that doesn't mean that the ISPs have fully assigned all their ...


11

That probably is about the only reason you would use the former construct, these days. The reason you're seeing this is probably that the default of ipv6only changed in nginx 1.3.4. Prior to that, it defaulted to off; in newer versions it defaults to on. This happens to interact with the IPV6_V6ONLY socket option on Linux, and similar options on other ...


10

For a truly small company, just wait for your ISP, and anyone else who handles your public face : if you have hosted mail, they should handle any IPv6 matters for mail. If someone hosts your website, ask them if they have made IPv6 DNS entries for you, or what their plan for doing so is. If they say they don't have one, I wouldn't worry about it much this ...


10

@yoonix has sent a link that might have a solution. Link-local, also known as APIPA. 169.254.0.0/16 - This is the "link local" block. As described in RFC3927, it is allocated for communication between hosts on a single link. Hosts obtain these addresses by auto-configuration, such as when a DHCP server cannot be found. If I were your customer, I'd ...


10

Each IPv4 address will take up 16 bytes in the reply. Each IPv6 address will take up 28 bytes in the reply. It is strongly recommended that you ensure the reply will fit in 512 bytes. That would allow for about 25 IPv4 addresses and 14 IPv6 addresses (considering that you need some other information in the packet as well). The exact limit depends on the ...



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