Addresses and Resolution

• Different at different layers.
• Two main concepts: link and network addressing.

Link layer (ethernet)

• Local addressing scheme.
• Assigned by vendors to cards.
• All cards on one 'wire' must have different MAC addresses.
• Valid on 'one wire' or 'one hub'.
• Not aware of tcp/ip workings.
• Switching: transferring information locally between separate 'wires' viewed as 'one wire'. translation tables.

Network layer (tcp/ip).

• global addressing scheme.
• Assigned by a human (you).
• All cards on the Internet as a whole must have different IP addresses.
• Valid world-wide.
• Not aware of link layer.
• Routing: transferring information non-locally.

Terminology

• Interface: one card or physical connection to the network.
• Single-home host: a host with one network interface card (NIC).
• Multi-home host: a host with two or more NIC's.

The game

• In the local domain, can only send messages to machines with known MAC address.
• In the global domain, can only send messages to machines with a known IP address.
• Humans refer to specific machines by name.
• Make all this jive by maintaining three databases.

Three parts of the game

• Map the local wire (ARP table)
• Map the network of wires (Routing table)
• Map from names to networks (Naming service)

Address translation table (ARP table)

• maps between local address (MAC/ethernet) and global address (ip/ipv6).
• maintained by each interface separately.
• describes map on one wire.
• arp -an: show the arp table.

  blackhole{couch}53: /usr/sbin/arp -a
  Net to Media Table
  Device   IP Address               Mask      Flags   Phys Addr 
  ------ -------------------- --------------- ----- ---------------
  le0    cb9000-2             255.255.255.255       00:20:9c:0f:0c:fe
  le0    conmoto              255.255.255.255       08:00:20:b0:51:fa
  le0    largo                255.255.255.255       08:00:20:79:c0:f1
  le0    forte                255.255.255.255       08:00:20:0e:04:39
  le0    blackhole            255.255.255.255 SP    08:00:20:7a:67:e9
  le0    conbrio              255.255.255.255       08:00:20:b0:17:0e
  le0    BASE-ADDRESS.MCAST.NET 240.0.0.0       SM    01:00:5e:00:00:00
  

• Device: which physical interface (determines which wire).
• Address: which machine we want.
• Mask: a binary mask showing how many addresses are mapped. If a bit in the mask is 0, then the corresponding bit in the original address can change. 255.255.255.255 means only one address.
• Phys Addr: the vendor address assigned to a device.
• Wait a minute: these "Addresses" are names, not IP addresses!
• arp command automatically converts them!

Routing table

• Tells one host how to send packets to another on the whole Internet.
• For each destination address, gives next hop host address.
• This is a neighbor of yours who knows more about how to send the packet to its real destination.
• If each machine forwards the packet to a neighbor nearer to its destination, then the packet will eventually reach its destination machine.
• To spy on routing, use netstat -rn:

   
  blackhole{couch}59: netstat -rn
  
  Routing Table:
    Destination           Gateway           Flags  Ref   Use   Interface
  -------------------- -------------------- ----- ----- ------ ---------
  130.64.23.0          130.64.23.164         U        3   1174  le0
  224.0.0.0            130.64.23.164         U        3      0  le0
  default              130.64.23.127         UG       0   5470  
  127.0.0.1            127.0.0.1             UH       01147795  lo0
  

• Destination: where you want to go.
• Gateway: next hop in getting there.
• Interface: which card to use.
• default: where to send anything you don't know about!

Protocols:

• ARP : address resolution protocol.
  • used all the time
  • works on link layer, makes machines aware of each other on network layer. machine 1: "hi there, i'm 08:00:2b:16:34:98 (MAC) and my IP address is 192.168.1.1 (tcp)" who is 192.168.1.10? broadcast: Where are you? machine 2: makes a note of this mapping in its 'arp table'. responds "I'm 08:00:2b:16:17:82 (MAC) and 192.168.1.10 (IP)." directed: I'm here!
• REVARP: reverse address resolution protocol.
  • used just in starting machines.
  • machine 1: who the heck am I, I'm 08:00:2b:16:17:82. broadcast: who am I?
  • server 2: I know you, you're 192.168.1.9. directed: you're so and so.
• BOOTP:
  • on top of User DataGram Protocol (UDP).
  • transaction:
    • machine 1 sends bootp broadcast 'who am i'
    • server (tcpip) responds not only with ip address, but with detailed network information. (where machine 1 is in network).
    • tells machine: ip address how big a network address space it's in . (254 node subnet). location of a default machine to send ip packets to. an arbitrary list of proprietary things local to machine.
    • requires centralized knowledge of hosts, and what they should have as configurations.
  • BOOTP has one drawback:
  • I must manually select your IP address.
  • if I have to select it,
    • difficult to manage large networks:
    • do something for each host in the network.
    • error prone.
    • Everything else in bootp is scalable: many machines from one configuration.
• DHCP: Dynamic Host Configuration Protocol:
  • rough superset of BOOTP.
  • keeps a pool of unused ip addresses, hands addresses out dynamically through a 'lease' concept.
  • bootp: 1 line per machine
  • dhcp: 1 line per TYPE of machine (pool).

Dynamic or Static addressing:

• dynamic addressing: host ip address can change upon reboot.
  • client host, no services, => no one has to remember your name.
  • lot of clients, => signficant management problem.
• static addressing: one ip address till end of time.
  • mail server: everyone needs to know your ip.
  • few servers => tractable management

Addressing conventions

• MAC:
  • addresses starting with 4,5,6,7 are user-assigned.
  • 0:0:0:0:0:0 and ff:ff:ff:ff:ff:ff are reserved (wire broadcast addresses)
  • all other addresses are vendor-assigned.
• IP:
  • addresses starting with 10., 192.168. are private and not routed on the backbone (RFC 1919).
  • addresses starting with other fields are assigned by your ISP or the American Registry of Internet Numbers (ARIN)
• Names:
  • All domain names are registered with some domain registry organization, subject to rules of the Internet Corporation for Assigned Names and Numbers (ICANN).
  • Organization delegates subdomains to particular sub-organizations within itself. E.g., tufts.edu allowed us to create eecs.tufts.edu..
  • machine names are assigned by you and tacked onto domain names. E.g., andante.eecs.tufts.edu.