feb 23
in the news
IP
Today we'll discuss IP addresses, IP routing, the CIDR/NAT systems commonly in use for end systems (home or campus), and DNS. So hold on to your hats ...
IP addressess
$ host www.mit.edu
www.mit.edu has address 18.9.22.169
32 bits , so 2**32 = 4 billion different addresses. Turns out that's not nearly enough.
CIDR : classless (historically different size blocks were "class A, class B, class C) InterDomain Routing.
(A silly acronym which turns out to mean "how to divide up which addresses are in my network".)
at home, last night
thirty:~$ ifconfig en1
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::21b:63ff:fecc:c553%en1 prefixlen 64 scopeid 0x6
inet 192.168.1.36 netmask 0xffffff00 broadcast 192.168.1.255
ether 00:1b:63:cc:c5:53
media: autoselect status: active
supported media: autoselect
my home router settings are :
modem IP address 192.168.1.1
subnet mask 255.255.255.0
DHCP start 192.168.1.15
DHCP end 192.168.1.47
lease time 1 day
The network in CIDR notation is 192.168.1.0/24
which means that any IP address with the
same 1st 24 bits is on the local network,
which means the last 8 bits can vary for machines on this network,
which means at most 2**8 = 256 different machines.
Actually, two less : the highest is "broadcast"; the lowest is "the network".
But at whatismyipaddress.com, it says I'm
64.222.112.248
How come?
NAT : Network Address Translation
The router changes each packet, modifying the private (192.168.*.*)
addresses to one that can be visible to the outside world.
Why? Many machines "inside"; few IP addresses needed for "outside".
Also gives more security/privacy.
Same on campus network.
-----------
thirty:~$ ifconfig en0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
ether 00:1b:63:96:20:2f
media: autoselect status: inactive
supported media: autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 10baseT/UTP <full-duplex,flow-control> 100baseTX <half-duplex> 100baseTX <full-duplex> 100baseTX <full-duplex,hw-loopback> 100baseTX <full-duplex,flow-control> 1000baseT <full-duplex> 1000baseT <full-duplex,hw-loopback> 1000baseT <full-duplex,flow-control> none
IP routing
Complicated; various tricky protocols for routers (computers
managed by networking companies, mostly) to figure out
where to send packets next. Mostly done with lookup tables
that get updated regularly based on network traffic.
ipv6
http://www.cyberciti.biz/faq/how-can-ipv6-address-used-with-webbrowser/
http://[2001:4860:0:2001::68]/
ipv6 : AAAA DNS record
... couldn't find for google, mit, dartmouth, marlboro
http://www.google.com/intl/en/ipv6/
thirty:~$ host ipv6.google.com
ipv6.google.com is an alias for ipv6.l.google.com.
ipv6.l.google.com has address 67.215.65.132
ipv6.l.google.com has IPv6 address 2001:4860:b009::63
ipv6.l.google.com has IPv6 address 2001:4860:b009::93
ipv6.l.google.com has IPv6 address 2001:4860:b009::68
ipv6.l.google.com has IPv6 address 2001:4860:b009::69
ipv6.l.google.com has IPv6 address 2001:4860:b009::67
ipv6.l.google.com has IPv6 address 2001:4860:b009::6a
firefox : http://[2001:4860:b009::63] … failed from home.
failed from on campus, too.
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DNS
A "phone-book" like lookup service mapping domain names (i.e. www.marlboro.edu) to IP addresses (206.192.68.7) and vice-versa.
(The reversed numbers are also stored in the same table, pointing back to the host name.)
$ host www.marlboro.edu
www.marlboro.edu is an alias for dubya-prod.marlboro.edu.
dubya-prod.marlboro.edu has address 206.192.68.7
$ host 206.192.68.7
7.68.192.206.in-addr.arpa domain name pointer dubya-prod.marlboro.edu.
thirty:~$
The DNS service stores different types of information :
- aliases (i.e. www.marlboro.edu is actually dubya-prod.marlboro.edu)
- mail servers (where should email go for marlboro.edu?)
- name servers (who knows which computer is foo.bar.marlboro.edu?)
It's a pyramid of information servers; each answers questions about its machines, stores answers received, and knows who is above to ask next. Like the IP routing system, which is similar.
There are 16 top level "root nameservers" :
misc
