Network Addressing



One thing all network nodes have in common is that in order for their addresses to be considered proper network addresses, they must have two parts present: a host address and a network address. The host address portion identifies the individual node. The network address portion identifies the network or network segment on which the host address can be found.

MAC addresses, which are Data Link layer addresses, differ from Netowkr layer addresses. MAC addresses are flat addresses in that they identify only the node with which they are associated and have no relationship to any other netowkr elements. In many ways, MAC addresses are like Social Security numbers for NICs, identifying only one NIC at a time.

A MAC address consists of a 48-bit or 6-byte hexadecimal number. It's represented in the form of six two-digit numbers separated by dashes. The first 24 bits of the MAC address contain a code assigned by the IEEE to uniquely identify the manufacturer of the card, and the 24 bits are a number uniquely assigned by the manufacturer.

Here are two points relating to MAC addresses that you're likely to see on the test:

  • ARP (Address Resolution Protocol): Uses broadcast messages to learn the MAC or Ethernet address of a known IP address. The node with the broadcasted IP address responds with its MAC address, completing the set.
  • Reverse ARP (RARP): Broadcasts messages to learn the IP address corresponding to a known MAC address. This requires the presence of a RARP server on the network. An example of where this procedure is used is booting a diskless workstation.

    • An IPX address is 10 bytes (80 bits) long. Four bytes is the network number and 6 bytes is the node number. The node number is normally the MAC address of the node.

    Remember
    Unlike IP addresses IPX addresses are not structured into classes. It's not uncommon for the network number not to use the entire 6 bytes. any zeros at the beginning of the network address are dropped.

    The 32-bit IP address is broken up into four octets, which are arranged into a dotted-decimal notation scheme. An octet is a set of 8 bits.

    Converting to Decimal

    The positions of the binary number represent powers of two:
    27   26   25   24   23   22   21   20

    Placing a 1 in one of the binary positions turns on that value and adds it into the decimal number representation. A zero in any position turns off the value in that position and it doesn't add into the number representation.

    Example 1

    Binary number 1111 1111
     27   26   25   24   23   22   21   20 
     1    1    1    1    1    1    1    1  
     128  64  32  16  8  4  2  1

    Result is: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

    Example 2

    Binary number 1011 1110
     27   26   25   24   23   22   21   20 
     1    0    1    1    1    1    1    0  
     128  0  32  16  8  4  2  0

    Result is: 128 + 0 + 32 + 16 + 8 + 4 + 2 + 0 = 190

    IP addresses are divided into five classes, each of which is designated with the alphabetic letters A to E. For the exam, forget about Classes D and E. Class D addresses are used for multicasting, and Class E addresses are reserved for testing and some mysterious future use.

    IP Address Class Assignments

    Class   First Octet Value  
    Class A   0-127
    Class B   128-191
    Class C   192-223

    Network and Host Representation by IP Address Class

    Class   Octet1   Octet2   Octet3   Octet4  
    Class A   Network Host Host Host
    Class B   Network Network Host Host
    Class C   Network Network Network Host

    Special IP Addresses

    Network Address Host Address Description Example
    0's 0's Default Cisco route 0.0.0.0
    0's Host address Local network hosts 0.0.0.115
    1's 1's Broadcast to local network 255.255.255.255
    Network Addresss 1's Broadcast to network address 192.21.12.255
    127 anything Loopback testing 127.0.0.1

    Subnet Mask

    The function of a subnet mask is to determine whether an IP address exists on the local network or whether it must be routed outside the local network. The subnet mask is applied to a message's destination address to extract the network address. If the extracted network address matches the local network ID, the destination is located on the local network. However, if they don't match, the message must be routed outside the local network. The process used to apply the subnet mask involves Boolean algebra to filter out non-matching bits to identify the network address.

    Working with subnet masks, you need only four basic principles of Boolean algebra:

  • 1 and 1 = 1
  • 1 and 0 = 0
  • 0 and 1 = 0
  • 0 and 0 = 0

    • The process of combining binary values with Boolean algebra is called anding.

    There are default standard subnet masks for Class A, B and C addresses. The commonly used subnet masks for each IP address class are:
    Address Class Subnet Mask
    Class A 255.0.0.0
    Class B 255.255.0.0
    Class C 255.255.255.0

    Verifying an IP Address

    IP addresses are verified using PING, Trace and Telnet. For the exam it's important that you know that PING is used to verify IP address connections to the Network layer and that Telnet is used to verify network IP address connections to the Application layer.

    Cisco PING Response Codes
    Response Meaning
     ! (exclamation mark) Success
     . (period) Timed out waiting for reply
     U Destination unreachable
     | (vertical bar) Ping process interrupted
     ? (question mark) Unknown packet type
     C Congestion-experienced
     & (ampersand) Time to live exceeded

    Cisco Trace Command Response Codes
    Response Meaning
     * Timed out
     !H router received packet but did not forward it
     N network unreachable
     P protocol unreachable
     U port unreachable



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