diff --git a/src/dnsmasq.h b/src/dnsmasq.h
index b66c58b..8bfff34 100644
--- a/src/dnsmasq.h
+++ b/src/dnsmasq.h
@@ -1117,8 +1117,9 @@ struct tftp_file {
 
 struct tftp_transfer {
   int sockfd;
+  u16 block_hi, ackprev;
   time_t retransmit, start;
-  unsigned int lastack, block, ackblock, blocksize, windowsize, timeout, expansion;
+  unsigned int lastack, block, blocksize, windowsize, timeout, expansion;
   off_t offset;
   union mysockaddr peer;
   union all_addr source;
diff --git a/src/tftp.c b/src/tftp.c
index 8f3768b..792288d 100644
--- a/src/tftp.c
+++ b/src/tftp.c
@@ -326,7 +326,8 @@ static void tftp_request(struct listener *listen, time_t now)
   transfer->start = now;
   transfer->backoff = 1;
   transfer->block = 1;
-  transfer->ackblock = 0; 
+  transfer->ackprev = 0;
+  transfer->block_hi = 0;
   transfer->blocksize = 512;
   transfer->windowsize = 1;
   
@@ -757,37 +758,38 @@ static void handle_tftp(time_t now, struct tftp_transfer *transfer, ssize_t len)
 	{
 	  /* Handle 16-bit block number wrap-around. */
 	  u16 new = ntohs(mess->block);
-	  u16 prev = transfer->ackblock;
-	  
-	  transfer->ackblock = new | (transfer->ackblock & 0xffff0000);
-	  
+	  u32 block;
+
 	  /* If the last ack received was in the top quarter of a 64k block
 	     and this one is in the bottom quarter, assume it has wrapped.
-
+	     
 	     Since this is UDP and an old packet can in theory wander in we may also
 	     need to drop back to a previous segment. Such an ACK is ignored below;
 	     here we're just getting the most likely 32 bit value from the
 	     16 bits that we have. */
-	  if (new <= 0x4000 && prev >= 0xc000)
-	    transfer->ackblock += 0x10000;
-	  else if (new >= 0xc000 && prev <= 0x4000 && transfer->ackblock != 0)
-	    transfer->ackblock -= 0x10000;
+	  if (new <= 0x4000 && transfer->ackprev >= 0xc000)
+	    transfer->block_hi++;
+	  else if (new >= 0xc000 && transfer->ackprev <= 0x4000 && transfer->block_hi != 0)
+	    transfer->block_hi--;
+
+	  transfer->ackprev = new;
+	  block = (((u32)transfer->block_hi) << 16) + (u32)new;
 	  
 	  /* Ignore duplicate ACKs and ACKs for blocks we've not yet sent. */
-	  if (transfer->ackblock >= transfer->lastack &&
-	      transfer->ackblock <= transfer->block) 
+	  if (block >= transfer->lastack &&
+	      block <= transfer->block) 
 	    {
 	      /* Got ack, move forward and ensure we take the (re)transmit path */
 	      transfer->retransmit = transfer->start = now;
 	      transfer->backoff = 0;
-	      transfer->lastack = transfer->ackblock + 1;
+	      transfer->lastack = block + 1;
 	      
 	      /* We have no easy function from block no. to file offset when
 		 expanding line breaks in netascii mode, so we update the offset here
 		 as each block is acknowledged. This explains why the window size must be
 		 one for a netascii transfer; to avoid  the block no. doing anything
 		 other than incrementing by one. */
-	      if (transfer->netascii && transfer->ackblock != 0)
+	      if (transfer->netascii && block != 0)
 		transfer->offset += transfer->blocksize - transfer->expansion;
 	    }
 	}