1) Solution is in the book 2) After B broadcasts any ARP query, all stations that had been sending to A's physical address will switch to sending to B's. A will see a sudden halt to all arriving traffic. (To guard against this, A might monitor for ARP broadcasts purportedly coming from itself; A might even immediately follow such broadcasts with its own ARP broadcast in order to return its traffic to itself. It is not clear, however, how often this is done.) If B uses self-ARP on startup, it will receive a reply indicating that its IP address is already in use, which is a clear indication that B should not continue on the network until the issue is resolved. 3) I = infinity a) Information | Distance to Reach Node Stored at Node | A B C D E F A 0 I 3 8 I I B I 0 I I 2 I C 3 I 0 I 1 6 D 8 I I 0 2 I E I 2 1 2 0 I F I I 6 I I 0 b) Information | Distance to Reach Node Stored at Node | A B C D E F A O I 3 8 4 9 B I 0 3 4 2 I C 3 3 0 3 1 6 D 8 4 3 0 2 I E 4 2 1 2 0 7 F 9 I 6 I 7 0 c) Information | Distance to Reach Node Stored at Node | A B C D E F A 0 6 3 6 4 9 B 6 0 3 4 2 9 C 3 3 0 3 1 6 D 6 4 3 0 2 9 E 4 2 1 2 0 7 F 9 9 6 9 7 0 4) Apply each subnet mask and if the corresponding subnet number matches the SubnetNumber column, then use the entry in Next-Hop. (In these tables there is always a unique match.) (a) Applying the subnet mask 255.255.255.128, we get 128.96.39.0. Use interface0 as the next hop. (b) Applying subnet mask 255.255.255.128, we get 128.96.40.0. Use R2 as the next hop. (c) All subnet masks give 128.96.40.128 as the subnet number. Since there is no match, use the default entry. Next hop is R4. (d) Next hop is R3. (e) None of the subnet number entries match, hence use default router R4. 5) (a) Q will receive three routes to P, along links 1, 2, and 3. (b) A -> B traffic will take link 1. B !A traffic will take link 2. Note that this strategy minimizes cost to the source of the traffic. (c) To have B -> A traffic take link 1, Q could simply be configured to prefer link 1 in all cases. The only general solution, though, is for Q to accept into its routing tables some of the internal structure of P, so that Q for example knows where A is relative to links 1 and 2. (d) If Q were configured to prefer AS paths through R, or to avoid AS paths involving links 1 and 2, then Q might route to P via R. 6) (a) Giving each department a single subnet, the nominal subnet sizes are 2^7, 2^6, 2^5, 2^5 respectively; we obtain these by rounding up to the nearest power of 2. A possible arrangement of subnet numbers is as follows. Subnet numbers are in binary and represent an initial segment of the bits of the last byte of the IP address; anything to the right of the / represents host bits. The / thus represents the subnet mask. Any individual bit can, by symmetry, be flipped throughout; there are thus several possible bit assignments. A 0/ one subnet bit, with value 0; seven host bits B 10/ C 110/ D 111/ The essential requirement is that any two distinct subnet numbers remain distinct when the longer one is truncated to the length of the shorter. (b) We have two choices: either assign multiple subnets to single departments, or abandon subnets and buy a bridge. Here is a solution giving A two subnets, of sizes 64 and 32; every other department gets a single subnet of size the next highest power of 2: A 01/ 001/ B 10/ C 000/ D 11/ 7) (a): B (b): A (c): E (d): F (e): C (f): D (For the last one, note that the first 14 bits of C4.6B and C4.68 match.) 8) (a) P's table: address nexthop C2.0.0.0/8 Q C3.0.0.0/8 R C1.A3.0.0/16 PA C1.B0.0.0/12 PB Q's table: address nexthop C1.0.0.0/8 P C3.0.0.0/8 R C2.0A.10.0/20 QA C2.0B.0.0/16 QB R's table: address nexthop C1.0.0.0/8 P C2.0.0.0/8 Q (b) The same, except for the following changes of one entry each to P's and R's tables: P: C3.0.0.0/8 Q //was R R: C1.0.0.0/8 Q //was P (c) Note the use of the longest-match rule to distinguish the entries for Q & QA in P's table, and for P & PA in Q's table. P's table: address nexthop C2.0.0.0/8 Q C2.0A.10.0/20 QA //for QA C1.A3.0.0/16 PA C1.B0.0.0/12 PB Q's table: address nexthop C1.0.0.0/8 P C1.A3.0.0/16 PA //for PA C2.0A.10.0/20 QA C2.0B.0.0/16 QB 9) (a) If Q does not advertise A to the world, then only traffic originating within Q will take the Q A link. Other traffic will be routed first to P. If Q does advertise A, then traffic originating at an external site B will travel in via Q whenever the B Q A path is shorter than the B P A path. (b) Q must advertise A s reachability to the world, but it may put a very low preference value on this link. (c) The problem is that most outbound traffic will take the DEFAULT path, and nominally this is a single entry. Some mechanism for load-sharing must be put into place. Alternatively, A could enter into its internal routing tables some of its most common external IP destinations, and route to these via Q. 10) For source D the tree's branches include 1) R1 and R2, 2) R4, 3) R5. For source E the tree's branched include 1) R7, 2) R6 and R3, 3) R6, R4 and R2. 11) (a) One multicast transmission involves all k +k^2 +...+k^(N-1) = (k^N - k)/(k - 1) links. (b) One unicast retransmission involves N links; sending to everyone would require N×k^N links. (c) Unicast transmission to x fraction of the recipients uses x × N × k^N links. Equating this to the answer in (a), we get x = (k^N - k)/((k - 1) × N × k^N) =(approx.) 1/(k - 1) × N 12) (a) correct (b) incorrect (::: is not defined as abbreviating notation) (c) incorrect (shorthand can only be used for one set of contiguous 0's) (d) correct (e) correct (this is shorthand for IPv4 address)