Flashcards
ARP (Address Resolution Protocol) / Layer 2
Flashcards do conjunto (13)
- 1What is the purpose of a MAC address?Identify the physical address of a host - its Network Interface Controller. So that L2 frames can be delivered and processed.
Media Access Control Address is used in local LAN networks to communicate with other devices. Every device can use the Broadcast MAC FF:FF:FF:... to contact every device in the local network, so every device can build up an ARP-Table which directly pinpoints MAC-Address with an IP. - 2On which OSI layer are MAC addresses used?Layer 2
- 3Name the fields in the ethernet header (IEEE 802.3)- Preamble, 7 Bytes
- SFD (Start of frame delimiter), 1 Byte
- Destination MAC address, 6 Bytes
- Source MAC Address, 6 Bytes
- Lenght, 2 Bytes
- Data, 46 - 1500 Bytes
- CRC, 4 Bytes - 4What is the purpose of the preamble?This is a pattern of alternative 0’s and 1’s which indicates starting of the frame and allow sender and receiver to establish bit synchronisation. Initially, PRE (Preamble) was introduced to allow for the loss of a few bits due to signal delays. But today’s high-speed Ethernet don’t need Preamble to protect the frame bits. PRE (Preamble) indicates the receiver that frame is coming and allow the receiver to lock onto the data stream before the actual frame begins.
- 5What is the purpose of the SFD?This is a 1-Byte field which is always set to 10101011. SFD indicates that upcoming bits are starting of the frame, which is the destination address. Sometimes SFD is considered the part of PRE, this is the reason Preamble is described as 8 Bytes in many places. The SFD warns station or stations that this is the last chance for synchronisation.
- 6What is the maximal length in the lenght field of the ethernet header?This 16-bit field can hold the length value between 0 to 65534, but length cannot be larger than 1500 because of some own limitations of Ethernet.
- 7What is the payload of a ethernet frame and how big is the minimal and maximal data in it?IP/TCP header and their own payload will be inserted here if Internet Protocol is used over Ethernet. The maximum data present may be as long as 1500 Bytes. In case data length is less than minimum length i.e. 46 bytes, then padding 0’s is added to meet the minimum possible length.
- 8What is the purpose of the CRC filed in the ethernet header?This field contains a 32-bits hash code of data, which is generated over the Destination Address, Source Address, Length, and Data field. If the checksum computed by destination is not the same as sent checksum value, data received is corrupted.
- 9How big is a ethernet frame?Size of frame of Ethernet IEEE 802.3 varies 64 bytes to 1518 bytes including data length (46 to 1500 bytes).
Note - Preamble and SFD are part of the first layer
Note2 - If 802.3q is implemented it would add 4 bytes for the tagging - 10How does ARP work?ARP is the Address Resolution Protocol which is used in the ARP-Table. The ARP-Table holds information over all local reachable hosts and their MAC-Address. The ARP-Table allows to lookup a certain IP ↔ MAC-Address Combo.
- 11How does ARP proxy work?Proxy ARP is a technique by which a proxy device on a given network answers the ARP queries for an IP address that is not on that network. The proxy is aware of the location of the traffic's destination, and offers its own MAC address as the (ostensibly final) destination. The traffic directed to the proxy address is then typically routed by the proxy to the intended destination via another interface or via a tunnel.
- 12What is the disadvantage of an ARP proxy?Disadvantage of Proxy ARP include scalability as ARP resolution by a proxy is required for every device routed in this manner, and reliability as no fallback mechanism is present, and masquerading can be confusing in some environments.
Proxy ARP can create DoS attacks on networks if misconfigured. For example, a misconfigured router with proxy ARP has the ability to receive packets destined for other hosts (as it gives its own MAC address in response to ARP requests for other hosts/routers), but may not have the ability to correctly forward these packets on to their final destination, thus black-holing the traffic.
Proxy ARP can hide device misconfigurations, such as a missing or incorrect default gateway. - 13Why are both IP and MAC address needed in a network?Ps are routed at some point, so if you want to leave the LAN. A MAC is needed to communicate on the LAN itself.
IP Addresses are logical and routable addresses. Computer A could potentially learn the IP Address of Computer 2. However, MAC Addresses are physical and are NOT routable. So, Computer A could not really learn the MAC Address of Computer 2.
And that’s why computers have both MAC Addresses and IP Addresses. MAC Addresses handle the physical connection from computer to computer while IP Addresses handle the logical routable connection from both computer to computer AND network to network.
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- Informatik FlashCards sobre ARP (Address Resolution Protocol) / Layer 2, criado por Simon Perez em 05-12-2019.
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- Simon Perez
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