CCNA v 1.1: Internet Layer Protocols

Internet Control Message Protocol (ICMP) — Provides control and feedback messages between IP devices. Address Resolution Protocol (ARP) — Using a destination IP address, ARP resolves or discovers the appropriate destination MAC (layer 2) address to use. Thus, ARP maps a Layer 3 address to a Layer 2 address.

Reverse Address Resolution Protocol (RARP) — Using a source MAC address, RARP retrieves an IP address form the RARP Server. RARP maps source Layer 2 address to a Layer 3 address; it is an early form of DHCP.

Dynamic Host configuration Protocol (DHCP) — DHCP is built on a client-server model, as follows:

  • The DHCP “server” allocates network addresses and delivers configuration parameters.
  • The DHCP “client” is a device that requests initialization parameters, including its IP address, from a DHCP server.

DHCP supports these three mechanisms for IP address allocation:

  1. Automatic — DHCP assigns a permanent IP address to a client.
  2. Dynamic — DHCP assigns an IP address to a client for a limited period of time (called a lease). Dynamic allocation is the only one that allows automatic reuse of an address that is no longer needed by the client to which it was assigned.
  3. Manual — A client IP address is assigned by the network administrator, and DHCP is used simply to convey the assigned address to the client.

Domain Name System (DNS) — Resolves domain names to IP addresses.

Internet Protocol (IP) — Provides the logical addressing structure and offers connectionless, best-effort delivery of packets (datagrams). IP version 4 (IPv4) and IP version 6 (IPv6) are described in the following sections.

IPv4

IPv4 Packet Header

Byte 1 Byte 2 Byte 3 Byte 4
Ver. IHL Type of Service Packet Length
Identification Flag Frag. Offset
Time to Live Protocol Header Checksum
Source Address
Destination Address
Options Padding

IPv4 Address Format

IPv4 addresses are 32 bits long. They are usually written in dotted decimal format: each 8-bit octet is written as a decimal number, and dots are put in between these numbers. Thus, when converting between binary and decimal for IPv4 addresses, always remember to convert 8 bits. An 8 bit number will have a decimal value between 0 and 255.

Bit Position* 7 6 5 4 3 2 1 0
Exponent Value 27 26 25 24 23 22 21 20
Decimal Value of Bit 128 64 32 16 8 4 2 1

*By convention bits are numbered starting at bit 0 = least-significant or right-most bit.

To convert a binary number to decimal, multiply each bit value in the number by its decimal value, and then sum the results. For example, to convert the 8-bit binary number 11010110 to decimal, do the following:

Bit Position* 1 1 0 1 0 1 1 0
Decimal Value of Bit 128 64 32 16 8 4 2 1
Bit Value x Decimal Value of Bit 128 64 0 16 0 4 2 0

The sum of these results is = 1*128 + 1*64 + 0*32 + 1*16 + 0*8 + 1*4 + 1*2 + 0*0 = 214. Therefore, the binary number 11010110 is 214 decimal.

Converting Decimal to Binary

To convert a decimal number (of 0 through 255) to binary, start at the left most bit (bit 7). If the decimal number is bigger than or equal to the decimal value of the bit, put a ‘1’ in the binary value of the number, and subtract the decimal value of the bit from the number. Otherwise put a ‘0’ in the binary value of the number. Repeat for the rest of the bits.

For example to convert 147:

    • Start at bit 7. Since 147 is bigger than or equal to 27 = 128, bit number 7 is a ‘1’. We now have 147 -128 = 19 remaining.
    • Since 19 is less than 26 = 64, bit number 6 is a ‘0’.
    • Since 19 is less than 25 = 32, bit number 5 is a ‘0’.
    • Since 19 is bigger than or equal to 24 = 16, bit number 4 is a ‘1’. We now have 19 – 16 = 3 remaining.
    • Since 3 is less than 23 = 8, bit number 3 is a ‘0’.
    • Since 3 is less than 22 = 4, bit number 2 is a ‘0’.
    • Since 3 is bigger than or equal to 21 = 2, bit number 1 is a ‘1’. We now have 3 – 2 = 1 remaining.
    • Since 1 is bigger than or equal to 20 = 1, bit number 0 is a ‘1’. We now have 1 – 1 = 0 remaining; this means we are finished converting.
    • Thus, the binary representation of 147 is 1 0 0 1 0 0 1 1.

Next week we’ll look at IPv4 classes.

Excerpted and available for download from Global Knowledge White Paper: CCNA v1.1 Exam Review: Critical Concepts of the 640-802 CCNA Exam

Resources:
CCNA Cert Check
CCNA Exam Prep Mobile App

Related Courses:
ICND1 — Interconnecting Cisco Network Devices 1
ICND2 — Interconnecting Cisco Network Devices 2
CCNAX — CCNA Boot Camp v1.1

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