Tag - MAC address

MAC Addressing – Framing and Duplex Types

A MAC address is hard-coded (burned in) on the network interface controller (NIC) of the Physical layer device attached to the network. Each MAC address must be unique and use the following format:

  • Consist of 48 bits (or 6 bytes).
  • Displayed by 12 hexadecimal digits (0 through 9, A through F).
  • First six hexadecimal digits in the address are a vendor code or organizationally unique identifier (OUI) assigned to that NIC manufacturer.
  • Last six hexadecimal digits are assigned by the NIC manufacturer and must be differ¬≠ent from any other number assigned by that manufacturer.

Example of a MAC address: 00:00:07:A9:B2:EB The OUI in this example is 00:00:07.

The broadcast address value is FFFF.FFFF.FFFF.

mac address

 

 Framing and Duplex Types

802.3 frame information and parameters are as follows:

  • The data-link header portion of the frame contains the Destination MAC address (6 bytes), Source MAC address (6 bytes), and Length (2 bytes).
  • The Logical Link Control portion of the frame contains Destination Service Access Point (DSAP), Source Service Access Point (SSAP), and Control information. All threeare 1 byte long. The Service Access Point (SAP) identifies an upper-layer protocol such as IP (06) or IPX (E0).
  • The Data and cyclical redundancy check (CRC) portion of the frame is also called the data-link trailer. The Data field can be anywhere from 43 to 1497 bytes long. The frame check sequence (FCS) field is 4 bytes long. FCS or CRC provides error detection.

Bridges and switches examine the source MAC address of each inbound frame to learn MAC addresses.

Switches are multiport bridges that use ASIC hardware chips for frame forwarding. Dedicatedbandwidth enables the switch port to guarantee the speed assigned to that port. For example, 100Mbps port connections get 100Mbps transmission rates.

Hubs use half-duplex technology. Switches can be set up for full duplex.

Basic CCNA Job Interview Questions

1: What is unicast and how does it work?

Unicast is a one-to-one transmission method. A single frame is sent from the
source to a destination on a network. When this frame is received by the switch,
the frame is sent on to the network, and the network passes the frame to its
destination from the source to a specific destination on a network.

ccna interview questions

2: What is multicast and how does it work?

** Multicast is a one-to-many transmission method. A single frame is sent from
the source to multiple destinations on a network using a multicast address. When
this frame is received by the switch, the frame is sent on to the network and the
network passes the frame to its intended destination group.

3:  What is broadcast and how does it work?

** Broadcast is a one-to-all transmission method. A single frame is sent from the
source to a destination on a network using a multicast address. When this frame
is received by the switch, the frame is sent on to the network. The network
passes the frame to all nodes in the destination network from the source to an
unknown destination on a network using a broadcast address. When the switch
receives this frame, the frame is sent on to all the networks, and the networks
pass the frame on to all the nodes. If it reaches a router, the broadcast frame is
dropped.

4: What is fragmentation?

** Fragmentation in a network is the breaking down of a data packet into smaller
pieces to accommodate the maximum transmission unit (MTU) of the network.

5: What is MTU? What’s the MTU for traditional Ethernet?

** MTU is the acronym for maximum transmission unit and is the largest frame
size that can be transmitted over a network. Messages longer than the MTU
must be divided into smaller frames. The network layer (Layer 3) protocol
determines the MTU from the data link layer (Layer 2) protocol and fragments the
messages into the appropriate frame size, making the frames available to the
lower layer for transmission without further fragmentation. The MTU for Ethernet
is 1518 bytes.

6: What is a MAC address?

** A MAC address is the physical address of a network device and is 48 bits (6
bytes) long. MAC addresses are also known as physical addresses or hardware
addresses.

7:  What is the difference between a runt and a giant, specific to traditional
Ethernet?

** In Ethernet a runt is a frame that is less than 64 bytes in length, and a giant is
a frame that is greater than 1518 bytes in length. Giants are frames that are
greater than the MTU used, which might not always be 1518 bytes.

8: What is the difference between store-and-forward and cut-through
switching?

** Cut-through switching examines just the frame header, determining the output
switch port through which the frame will be forwarded. Store-and-forward
examines the entire frame, header and data payload, for errors. If the frame is
error free, it is forwarded out its destination switch port interface. If the frame has
errors, the switch drops the frame from its buffers. This is also known as
discarding the frame to the bit bucket.

9: What is the difference between Layer 2 switching and Layer 3 switching?

* * Layer 2 switches make their forwarding decisions based on the Layer 2 (data
link) address, such as the MAC address. Layer 3 switches make their forwarding
decisions based on the Layer 3 (network) address.

10: What is the difference between Layer 3 switching and routing?

** The difference between Layer 3 switching and routing is that Layer 3 switches
have hardware to pass data traffic as fast as Layer 2 switches. However, Layer 3
switches make decisions regarding how to transmit traffic at Layer 3 in the same
way as a router. A Layer 3 switch cannot use WAN circuits or use routing
protocols; a router is still required for these functions.

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What features or restrictions can a DHCP server have

While the DHCP server protocol is designed to support dynamic management of IP addresses, there is nothing to stop someone from implementing a server that uses the DHCP protocol, but does not provide that kind of support. In particular, the maintainer of a BOOTP server-implementation might find it helpful to enhance their BOOTP server to allow DHCP clients that cannot speak “BOOTP” to retrieve statically defined addresses via DHCP. The following terminology has become common to describe three kinds of IP address allocation/management. These are independent “features”: a particular server can offer or not offer any of them:

  • Manual allocation: the server’s administrator creates a configuration for the server that includes the MAC address and IP address of each DHCP client that will be able to get an address: functionally equivalent to BOOTP though the protocol is incompatible.
  • Automatic allocation: the server’s administrator creates a configuration for the server that includes only IP addresses, which it gives out to clients. An IP address, once associated with a MAC address, is permanently associated with it until the server’s administrator intervenes.
  • Dynamic allocation: like automatic allocation except that the server will track leases and give IP addresses whose lease has expired to other DHCP clients.

Other features which a DHCP server may or may not have:

  • Support for BOOTP clients.
  • Support for the broadcast bit.
  • Administrator-settable lease times.
  • Administrator-settable lease times on manually allocated addresses.
  • Ability to limit what MAC addresses will be served with dynamic addresses.
  • Allows administrator to configure additional DHCP option-types.
  • Interaction with a DNS server. Note that there are a number of interactions that one might support and that a standard set & method is in the works.
  • Interaction with some other type of name server, e.g. NIS.
  • Allows manual allocation of two or more alternative IP numbers to a single MAC address, whose use depends upon the gateway address through which the request is relayed.
  • Ability to define the pool/pools of addresses that can be allocated dynamically. This is pretty obvious, though someone might have a server that forces the pool to be a whole subnet or network. Ideally, the server does not force such a pool to consist of contiguous IP addresses.
  • Ability to associate two or more dynamic address pools on separate IP networks (or subnets) with a single gateway address. This is the basic support for “secondary nets”, e.g. a router that is acting as a BOOTP relay for an interface which has addresses for more than one IP network or subnet.
  • Ability to configure groups of clients based upon client-supplied user and/or vendor class. Note: this is a feature that might be used to assign different client-groups on the same physical LAN to different logical subnets.
  • Administrator-settable T1/T2 lengths.
  • Interaction with another DHCP server. Note that there are a number of interactions that one might support and that a standard set & method is in the works.
  • Use of PING (ICMP Echo Request) to check an address prior to dynamically allocating it.
  • Server grace period on lease times.
  • Ability to force client(s) to get a new address rather than renew.

Following are some features related not to the functions that the server is capable of carrying out, but to the way that it is administered.

  • Ability to import files listing manually allocated addresses (as opposed to a system which requires you to type the entire configuration into its own input utility). Even better is the ability to make the server do this via a command that can be used in a script, rdist, rsh, etc.
  • Graphical administration.
  • Central administration of multiple servers.
  • Ability to import data in the format of legacy configurations, e.g. /etc/bootptab as used by the CMU BOOTP daemon.
  • Ability to make changes while the server is running and leases are being tracked, i.e. add or take away addressees from a pool, modify parameters.
  • Ability to make global modifications to parameters, i.e., that apply to all entries; or ability to make modifications to groups of ports or pools.
  • Maintenance of a lease audit trail, i.e. a log of the leases granted.

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