CS 6250 COMPUTER NETWORKS EXAM |
STUDY QUESTIONS AND ANSWERS |
LATEST 2025/2026
Why does NAT violate the end to end argument? - Correct Answer -
Devices are not globally addressable, so other devices can't initiate
inbound connections.
ARP (definition) - Correct Answer - Address Reolution Protocol.
ARP table maps IP addresses to MAC addresses
ARP (process) - Correct Answer - 1. Host queries with IP address -
broadcast.
2. Host with IP address responds with MAC address - unicast.
3. When host receives reply, adds to ARP table.
4. To send packets, host puts IP packet inside Ethernet frame using
MAC address destination.
Hubs vs. Switches - Correct Answer - Hub: Broadcast medium where all
packets on the network are seen everywhere.
,Switch: Partitions LAN into separate broadcast domains / LAN
segments. Frames bound for a host in the same segment aren't
forwarded to other segments.
Switch Table (definition) - Correct Answer - Maps destination MAC
address to switch output ports
Learning Switches (process) - Correct Answer - 1. Switch receives a
packet with a destination address.
2a. If there is an entry in the table, forward accordingly and we are done.
Otherwise...
2b. Flood and take note of source address + switch port in table.
Broadcast Storms + How to Prevent - Correct Answer - Broadcast
storms can occur when there is a loop in the network topology, and
packets keep getting forwarded.
Prevent by creating a fowarding tree (spanning tree).
Spanning Tree (definition) - Correct Answer - A loop-free topology that
contains every node.
Instead of flooding, a switch forwards packets along the spanning tree.
Spanning Tree (construction overview) - Correct Answer - 1. Elect a root
(typically switch with smallest ID)
, 2. At each switch, exclude a link if it's not on the shortest path to the root.
Spanning Tree (process per node) - Correct Answer - 1. Every node
starts by thinking it is the root, broadcasting this information.
2. If a node learns of another node with a smaller ID, it updates its view
of the root and computes the distance to the new root, sending this
information to other nodes as well.
3. This process repeats until you end up with a spanning tree.
Switches vs. Routers - Correct Answer - Switches operate on layer 2
(ethernet). They are auto-configuring, and forwarding tends to be fast.
Routers operate on layer 3 (IP). Topologies are not restricted to a
spanning tree, and they support multipath routing (a packet can be sent
through multiple possible paths).
Layer 2 vs. Layer 3 Switching - Correct Answer - Layer 2 switching is
more convenient, but its major limitation is broadcast. Spanning tree
messages and ARP queries impose a high load on the network.
Buffer Sizing (simple / rule of thumb) - Correct Answer - If RTT (round
trip time) = 2T and capacity of bottleneck = C, then the buffer size should
be 2T*C.
STUDY QUESTIONS AND ANSWERS |
LATEST 2025/2026
Why does NAT violate the end to end argument? - Correct Answer -
Devices are not globally addressable, so other devices can't initiate
inbound connections.
ARP (definition) - Correct Answer - Address Reolution Protocol.
ARP table maps IP addresses to MAC addresses
ARP (process) - Correct Answer - 1. Host queries with IP address -
broadcast.
2. Host with IP address responds with MAC address - unicast.
3. When host receives reply, adds to ARP table.
4. To send packets, host puts IP packet inside Ethernet frame using
MAC address destination.
Hubs vs. Switches - Correct Answer - Hub: Broadcast medium where all
packets on the network are seen everywhere.
,Switch: Partitions LAN into separate broadcast domains / LAN
segments. Frames bound for a host in the same segment aren't
forwarded to other segments.
Switch Table (definition) - Correct Answer - Maps destination MAC
address to switch output ports
Learning Switches (process) - Correct Answer - 1. Switch receives a
packet with a destination address.
2a. If there is an entry in the table, forward accordingly and we are done.
Otherwise...
2b. Flood and take note of source address + switch port in table.
Broadcast Storms + How to Prevent - Correct Answer - Broadcast
storms can occur when there is a loop in the network topology, and
packets keep getting forwarded.
Prevent by creating a fowarding tree (spanning tree).
Spanning Tree (definition) - Correct Answer - A loop-free topology that
contains every node.
Instead of flooding, a switch forwards packets along the spanning tree.
Spanning Tree (construction overview) - Correct Answer - 1. Elect a root
(typically switch with smallest ID)
, 2. At each switch, exclude a link if it's not on the shortest path to the root.
Spanning Tree (process per node) - Correct Answer - 1. Every node
starts by thinking it is the root, broadcasting this information.
2. If a node learns of another node with a smaller ID, it updates its view
of the root and computes the distance to the new root, sending this
information to other nodes as well.
3. This process repeats until you end up with a spanning tree.
Switches vs. Routers - Correct Answer - Switches operate on layer 2
(ethernet). They are auto-configuring, and forwarding tends to be fast.
Routers operate on layer 3 (IP). Topologies are not restricted to a
spanning tree, and they support multipath routing (a packet can be sent
through multiple possible paths).
Layer 2 vs. Layer 3 Switching - Correct Answer - Layer 2 switching is
more convenient, but its major limitation is broadcast. Spanning tree
messages and ARP queries impose a high load on the network.
Buffer Sizing (simple / rule of thumb) - Correct Answer - If RTT (round
trip time) = 2T and capacity of bottleneck = C, then the buffer size should
be 2T*C.
