Network layer subnetting and control protocols
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Network Layer – Subnetting and Control Protocols. Dr. Sanjay P. Ahuja, Ph.D. Fidelity National Financial Distinguished Professor of CIS School of Computing, UNF. Example of Subnetting. R1 R2

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Network Layer – Subnetting and Control Protocols

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Network Layer – Subnetting and Control Protocols

Dr. Sanjay P. Ahuja, Ph.D.

Fidelity National Financial Distinguished Professor of CIS

School of Computing, UNF

Example of Subnetting R1 R2

Lan 1 Lan 2 Lan 3

H1 H2 H3

Lan 1

Subnet #

Subnet Mask

Lan 2

Subnet #

Subnet Mask

Lan 3

Subnet #

Subnet Mask


Example of Subnetting

  • The bitwise AND of a host IP address and the subnet mask provides the subnet # of the host and of all other hosts on the same subnet.

  • Say H1 wants to send a packet to H2. H1 does a bitwise AND between its own subnet mask and the destination IP address.

    • AND = =

      This result does not match the subnet # for H1 and so H1 knows H2 is on a different subnet.

      To support subnetting, the routing table of a router holds entries of the form <subnet #, subnet mask, next hop>

      Router R1’s table:

Internet Control Protocols in the Network Layer - ICMP

  • Internet Control Message Protocol (ICMP) is a companion protocol to IP.

  • ICMP defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP datagram successfully.

  • Examples of error messages:

  • Destination Unreachable: Could be due to link failure or router unable to locate destination

  • Time Exceeded: When TTL decrements to 0.

  • ICMP – Redirect: Tells source host that there is a better route to the destination.

  • ECHO REQUEST: To see if a destination is alive and reachable.


  • Source Quench: Used to throttle hosts that were sending too many packets.

Internet Control Protocols in the Network Layer - ARP

  • Address Resolution Protocol (ARP)

    An IP address (4-bytes) needs to be mapped into a link-level address (e.g. a 6-bytes Ethernet or MAC address) before a packet encapsulated in a frame can be delivered to a host on a network (LAN) or to a router on a LAN that will forward it.

  • A source host on a LAN broadcasts an ARP REQUEST packet with the IP address of the destination host (“Who owns this IP address?”) and the destination host replies with an ARP REPLY packet containing its MAC address. The source host caches the destination MAC address for future use.

  • Then the source host sends the MAC/Ethernet frame containing the IP packet to the destination host.

  • To allow mappings to change (e.g. if an Ethernet NIC card breaks or is replaced) entries in the ARP cache are timed out every 15 minutes.

Internet Control Protocols in the Network Layer – RARP, BOOTP, DHCP

  • Reverse Address Resolution Protocol (RARP)

    This answers the reverse question, i.e. given an Ethernet address, what is the corresponding IP address. Used when booting a diskless workstation on the LAN.

  • Upon boot-up, RARP allows a workstation to broadcast its Ethernet address and ask if any one knows its IP address (“Does anyone know my IP address?”) The RARP server responds with the IP address of the workstation.

  • A disadvantage of RARP is that it uses a destination address of all 1s (limited broadcasting) to reach the RARP server. However, such broadcasts are not forwarded by routers, so a RARP server is needed on each LAN in an organization.

  • Once a workstation has its IP address it can contact the file server for the image of the OS, file server’s IP address, default router’s IP address. Each of these requests is a separate request-reply. This is inefficient.

Internet Control Protocols in the Network Layer – RARP, BOOTP, DHCP

  • Boot Protocol (BOOTP)

    Uses UDP. It does a limited broadcast which is forwarded over routers. Only one BOOTP server is needed for all the LANs in an organization.

  • To improve on efficiency, BOOTP server responds with the workstation’s IP address, default router’s IP address, file server’s IP address, and filename of the OS image in one reply packet.

  • BOOTP does not automatically assign IP addresses to a client workstation. The MAC address – IP address mapping table is still required to be configured and maintained manually at the BOOTP server.

  • So when a new host is added to a LAN, it cannot use BOOTP until a network admin has assigned it an IP address and entered its Ethernet address and IP address into the BOOTP configuration table manually.

Internet Control Protocols in the Network Layer – RARP, BOOTP, DHCP

  • Dynamic Host Configuration Protocol (DHCP)

    An IP address can be automatically assigned to a client host from a pool of IP addresses. IP addresses assigned automatically are leased for a fixed amount of time.

  • DHCP allows both manual IP address and automatic assignment and has replaced both RARP and BOOTP.

  • The DHCP server need not be on the same LAN as the requesting client host. Since the DHCP server may not be reachable by broadcasting, a DHCP relay agent is needed on each LAN.

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