Term
| What does IPv6 use instead of ARP? |
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Definition
| Neighbour Discovery Protocol |
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Term
| What is RARP and where was it used? |
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Definition
| RARP provides the reverse mapping from ARP. It was used by diskless systems and is rarely used today. |
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Term
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Definition
| ARP provides a dynamic mapping between IPv4 addresses and the hardware addresses (used by various technologies) for devices on the same network segment. |
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Term
| On what type of networks is ARP used? |
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Definition
| Broadcast networks. NBMA networks require more complex mapping protocols. |
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Term
| Briefly describe how ARP works, if no matching record is found in the cache. |
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Definition
a. ARP broadcasts a request to the whole link layer segment.
b. The host with a matching IP responds with a unicast reply and updates its cache with the senders details.
c. All other hosts ignore the request.
d. The original sender updates its cache and sends the application traffic to the correct destination. |
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Term
| For how long will the ARP cache keep an entry? |
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Definition
20 mins - completed entry
3 mins incomplete entry |
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Term
| Where within an Ethernet frame will an ARP message be found? |
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Definition
| After the Length/Type field, assuming there is no 802.1p/q tag. |
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Term
| Describe the fields of an ARP message. |
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Definition
Fixed size fields:
a. Hard Type - 2 bytes - Hardware address type (1 for Eth)
b. Prot Type - 2 bytes - Protocol type (0x0800 for IPv4)
c. Hard Size - 1 bytes - Address size in bytes (6 for Eth)
d. Prot Size - 1 bytes - Address size in bytes (4 for IPv4)
e. Op - 2 bytes - Specifies specific ARP message
Variable size fields
a. Sender's HW address - same in Eth and ARP message.
b. Sender's protocol address
c. Target HW address - set to 0 for ARP request
d. Target protocol address
Padding - Eth frame must be 64 bytes min. |
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Term
| What happens if an ARP request for a non-existent host is broadcast? |
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Definition
| The request will be retransmitted before the host gives up. The time between request depends on the machine and traffic type. |
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Term
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Definition
| Proxy ARP allows a system to answer ARP requests for a diffent host. Proxy ARP is not commonly used and should be avoided where possible. |
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Term
| What is gratuitous ARP and what are the benefits? |
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Definition
Gratuitous ARP occurs when a host sends an ARP request looking for its own address. Benefits are as follows:
a. It lets a host determine if another host is using the same IPv4 address i.e. it will get an unexpected reply.
b. If a host has changed its hardware/IPv4 address, a gratuitous ARP request will inform other hosts of the change and they will update their cache. |
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Term
| What is Address Conflict Detection (ACD)? |
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Definition
| ACD determines if an IPv4 address is being used by another host by sending probes/announcements. Probes are similar to gratuitous ARP but they will not pollute the caches of other hosts if the address is being used. |
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Term
| Where might link-layer broadcast ARP replies be used? |
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Definition
| They can be used with ACD to allow all hosts to invalidate their caches during a conflict. However, all hosts will have to process all replies. |
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