chapter 21 -- ip: internet addressing

(rev. 01/20/2010)

Notes On Chapter Twenty-one -- IP: Internet Addressing

21.1 Introduction
- Beginning of description of Internet protocol software
- Addressing scheme (IPv4)
- Address masks
- Classless and subnet addressing
21.2 Addresses for the Virtual Internet
- The Internet is a virtual network existing at a level of abstraction above the physical networks that support it.
- To maintain the illusion of the Internet - a single virtual network that connects computers on a multiplicity of real physical networks, the designers of the Internet chose a uniform addressing scheme, packet format, and delivery technique.
- The design of Internet addresses and their assignment to NIC's is independent of the addressing scheme used on the physical networks.
21.3 The IP Addressing Scheme
- An Internet Protocol address (aka IP address, aka Internet address) is a 32-bit number.
- Packets traveling across the Internet are supposed to contain two IP addresses: that of the sender (the source address) and that of the intended recipient (the destination address).
- Example: The IP address of castor.csustan.edu is 130.17.70.83. Each of the parts of the address stands for an 8-bit number ( 4*(8 bits)=32 bits )
21.4 The IP Address Hierarchy
- Each 32-bit IP address has two parts: a prefix and a suffix.
- A prefix is assigned by an authority to one network.
- All hosts on a given network have the same prefix.
- Example: the prefix for the csustan network is 130.17. All computers on our campus network have IP numbers that start with 130.17.
- It is very important to NOT assign the same IP address to two different NIC's. Since the prefixes of two computers on the same network have to be the same, that means the suffixes have to be different.
- Example: castor.csustan.edu: 130.17.70.83
- Example: pollux.csustan.edu: 130.17.70.15
21.5 Original Classes of IP Addresses
- In the original IP addressing scheme, there were five different classes of address:
  - Refer to figure 21.1 on page 348.
  - Class A: first bit is 0, prefix is 8 bits, suffix is 24 bits - provides addresses for about 128 big networks with many hosts.
  - Class B: first two bits are 10, prefix is 16 bits, suffix is 16 bits - provides addresses for about 16,000 medium-sized networks with up to about 64,000 hosts
  - Class C: first three bits are 110, prefix is 24 bits, suffix is 8 bits - provides addresses for about two million small networks with up to about 256 hosts.
  - Class D: first four bits are 1110 - used for multicast addresses
  - Class E: first four bits are 1111 - reserved (not assigned)
21.6 Dotted Decimal Notation
- Refer to figure 21.2 on page 349.
- Example: castor.csustan.edu has IP address 130.17.70.83
```
	 130 = 128+2     = 1000 0010 
	 17  = 16+1      = 0001 0001 
	 70  = 64+4+2    = 0100 0110 
	 83  = 64+16+2+1 = 0101 0011 
```
  So the binary equivalent of 130.17.70.83 is the 32-bit number:
```
	 1000 0010 0001 0001 0100 0110 0101 0011 
```
- Note that, since they represent unsigned 8-bit binary numbers, each of the four sections of a dotted decimal is in the range 0 to 255. It can't be any larger than 255 (or less than 0).
21.7 Division of the Address Space
- Refer to figure 21.3 on page 350.
- See info on classes A, B, C above in section 21.5
21.8 Authority for Addresses
- A company or organization usually gets its network prefix(es) assigned to it by an ISP.
- There are registrars who make blocks of addresses available to ISPs.
- The Internet Corporation for Assigned Names and Numbers (ICANN) authorizes the registrars.
- ICANN has the ultimate responsibility for assignment of IP addresses and adjudication of disputes related to IP address assignments.
21.9 Subnet and Classless Addressing
- Refer to figure 21.4 on page 351.
- The boundaries chosen for class A, B and C addresses were 'unfortunate' in some ways. Class C provided for too few computers, Class A and class B addresses were relatively few in number and demand eventually exceeded supply.
- To deal with this problem, schemes involving dividing IP addresses at arbitrary bit boundaries were employed.
- These schemes are variously called subnet addressing or classless addressing.
- This generalized A, B, and C addresses with their 8-, 16-, and 24-bit prefixes. Now there could also be, for example, 17-, 18-, 19-, 20-, 21-, 22- and 23-bit prefixes.
- This allowed assigned address ranges to more closely match the needs of the networks.
21.10 Address Masks
- In the original addressing scheme, software could look at the first few bits of an address and determine its class and the location of the boundary between the prefix and the suffix.
- For example, if the first three bits of a classful address are 110, it is a class C address and therefore the first 24 bits are the prefix and the last 8 are the suffix.
- When the classless addressing scheme was adopted, it had the effect of chopping up class A, B and C addresses in new ways - so that one could no longer tell where the prefix/suffix boundary was by examining the first few bits of the address.
- Sometimes an address mask is used to indicate the location of the prefix/suffix boundary.
- The example in the book can be used to illustrate:
```
    1000 0000 0000 1010 0000 0010 0000 0011  (destination address)
    1111 1111 1111 1111 0000 0000 0000 0000  (address mask)
    
```
  The boundary between the 1's and 0's in the mask can be used to signify that the prefix/suffix boundary in the destination address is that same location.
- Certain 'bit operations' are easy for computers to perform. Routers use address masks and bit operations to efficiently test destination addresses to see how they need to be routed.
```
    N==(D&M)
```
21.11 CIDR Notation
- In the CIDR notation, the prefix/suffix boundary may be expressed just with a slash and number added to the IP address.
- For example: 192.5.48.69/26 means that the IP address is 192.5.48.69 and the prefix has 26 bits.
21.12 A CIDR Example
- Refer to figure 21.4 on page 354.
21.13 CIDR Host Addresses
- Refer to figure 21.6 on page 355.
21.14 Special IP Addresses
- Some IP addresses are reserved - they can't be assigned to hosts because they are used for other purposes:
  - 21.14.1 Network Address: The result of adjoining a suffix of "all zeros" to a network prefix. It denotes the network itself. Example: 130.17.0.0 denotes the csustan.edu network.
  - 21.14.2 Directed Broadcast Address: The result of adjoining a suffix of "all ones" to a network prefix. This address can be used from outside the network to send a copy of a packet to every host on the network. Example: 130.17.255.255
  - 21.14.3 Limited Broadcast Address This is the IP address 255.255.255.255 - all bits are 1's. This address is used by a host to send a packet to every computer on the directly-connected network. A booting system that has not yet learned anything about the network it is connected to may use the limited broadcast address to send out a request for information.
  - 21.14.4 This Computer Address: There is an IP protocol a booting computer can use to broadcast a request that asks: "Will somebody please tell me my IP address?" The sending computer has to put something in the packet field for the source address. The IP address 0.0.0.0 is used for this purpose. We might say that 0.0.0.0 means "me".
  - 21.14.5 Loopback Address: Any IP address of the form 127.x.y.z is a loopback address. A loopback address is used to test networking software. Packets sent out the loopback interface go down the protocol stack and back up again, but are not transmitted onto the network.
21.15 Summary of Special IP Addresses
- Refer to figure 21.7 on page 358.
21.16 The Berkeley Broadcast Address
- Many years ago,developers of early TCP/IP software made an error, causing the protocol software to use network addresses as if they were directed broadcast addresses. As a result, some compatibility problems continue to this day.
21.17 Routers and the IP Addressing Principle
- Refer to figure 21.8 on page 359.
- Although we often think of an IP address as being assigned to a computer, in reality an IP addresss is assigned to a connection between a computer and a network.
- Since an Internet router has connections to at least two different networks, it needs at least two IP addresses.
- There's no requirement that the host parts of a router's IP addresses match. It's OK if they are the same, but they don't have to be.
21.18 Multi-Homed Hosts
- Just as a router needs multiple IP addresses, so does a multi-homed host.