diff --git a/client/inc/nb65_constants.i b/client/inc/nb65_constants.i
index 719ebdd..5563967 100644
--- a/client/inc/nb65_constants.i
+++ b/client/inc/nb65_constants.i
@@ -6,14 +6,6 @@ NB65_DISPATCH_VECTOR            = $800d
 NB65_PERIODIC_PROCESSING_VECTOR = $8010
 NB65_VBL_VECTOR                 = $8013
 
-;offsets in IP packet of various interesting bits:
-NB65_IP_SRC	          = 12  ;offset of "source address" field in an IP packet header
-NB65_IP_DEST		      = 16  ;offset of "destination address" field in an IP packet header
-NB65_UDP_SRC_PORT	    = 20  ;offset of udp source port field in ip packet
-NB65_UDP_DEST_PORT    = 22  ;offset of udp source port field in ip packet
-NB65_UDP_DATA_LENGTH  = 24  ;offset of length field in udp packet
-NB65_UDP_DATA	        = 28  ;offset of data in udp packet
-
 ;offsets in NB65 configuration structure
 NB65_CFG_MAC        = $00     ;6 byte MAC address
 NB65_CFG_IP         = $06     ;4 byte local IP address (will be overwritten by DHCP)
@@ -62,7 +54,8 @@ NB65_DNS_RESOLVE_HOSTNAME     =$07 ;inputs: AX points to a DNS parameter structu
                                    ;NB65_DNS_HOSTNAME_IP updated with IP address corresponding to hostname.
 NB65_UDP_ADD_LISTENER         =$08 ;inputs: AX points to a UDP listener parameter structure, outputs: none
 NB65_GET_INPUT_PACKET_INFO    =$09 ;inputs: AX points to a UDP packet parameter structure, outputs: UDP packet structure filled in
-NB65_UNHOOK_VBL_IRQ           =$0A ;inputs: none, outputs: none (removes call to NB65_VBL_VECTOR on IRQ chain)
+NB65_SEND_UDP_PACKET          =$0A ;inputs: AX points to a UDP packet parameter structure, outputs: none packet is sent
+NB65_UNHOOK_VBL_IRQ           =$0B ;inputs: none, outputs: none (removes call to NB65_VBL_VECTOR on IRQ chain)
 
 NB65_PRINT_ASCIIZ             =$80 ;inputs: AX= pointer to null terminated string to be printed to screen, outputs: none
 NB65_PRINT_HEX                =$81 ;inputs: A = byte digit to be displayed on screen as (zero padded) hex digit, outputs: none
diff --git a/client/ip65/function_dispatcher.s b/client/ip65/function_dispatcher.s
index 3f53d81..df991c4 100644
--- a/client/ip65/function_dispatcher.s
+++ b/client/ip65/function_dispatcher.s
@@ -26,7 +26,12 @@
 .import udp_add_listener
 .import ip_inp
 .import udp_inp
-
+.import udp_send
+.import udp_send_src
+.import udp_send_src_port
+.import udp_send_dest
+.import udp_send_dest_port
+.import udp_send_len
 .import copymem
 .import cfg_mac
 .importzp copy_src
@@ -227,18 +232,59 @@ irq_handler_installed:
   sta (nb65_params),y
   
   iny
-  lda #<udp_inp+8 ;payload ptr (lo byte)
+  lda #<(udp_inp+8) ;payload ptr (lo byte)
   sta (nb65_params),y
 
   iny
-  lda #>udp_inp+8 ;payload ptr (hi byte)
+  lda #>(udp_inp+8) ;payload ptr (hi byte)
   sta (nb65_params),y
 
   clc
   rts
 :  
 
+  cpy #NB65_SEND_UDP_PACKET
+  bne :+
+  ldy #3
+@copy_dest_ip:  
+  lda (nb65_params),y
+  sta udp_send_dest,y
+  dey
+  bpl @copy_dest_ip
+  
+  ldy #NB65_REMOTE_PORT  
+  lda (nb65_params),y
+  sta udp_send_dest_port
+  iny
+  lda (nb65_params),y
+  sta udp_send_dest_port+1
+  iny
 
+  lda (nb65_params),y
+  sta udp_send_src_port
+  iny
+  lda (nb65_params),y
+  sta udp_send_src_port+1
+  iny
+
+
+  lda (nb65_params),y
+  sta udp_send_len
+  iny
+  lda (nb65_params),y
+  sta udp_send_len+1
+  iny
+
+  ;AX should point at data to send
+  lda (nb65_params),y
+  pha
+  iny
+  lda (nb65_params),y  
+  tax
+  pla
+  jmp udp_send
+
+:  
   cpy #NB65_UNHOOK_VBL_IRQ
   bne :+
   ldax  jmp_old_irq+1
diff --git a/client/test/test_cart_api.s b/client/test/test_cart_api.s
index 3d61465..2eff616 100644
--- a/client/test/test_cart_api.s
+++ b/client/test/test_cart_api.s
@@ -23,7 +23,7 @@
   print_a = $ffd2
     
   .zeropage
-temp_ptr:		.res 2
+  temp_ptr:		.res 2
   
   .bss
   nb65_param_buffer: .res $20  
@@ -202,12 +202,13 @@ udp_callback:
   lda nb65_param_buffer+NB65_PAYLOAD_LENGTH
   ldy #NB65_PRINT_HEX
   jsr NB65_DISPATCH_VECTOR
-  
+  print_cr  
   ldax #data
   ldy #NB65_PRINT_ASCIIZ
   jsr NB65_DISPATCH_VECTOR
 
   ldax nb65_param_buffer+NB65_PAYLOAD_POINTER
+  
   stax temp_ptr
   ldx nb65_param_buffer+NB65_PAYLOAD_LENGTH ;assumes length is < 255
   ldy #0
@@ -216,12 +217,27 @@ udp_callback:
   jsr print_a
   iny
   dex
-  bne :-
+  bpl :-
   
   print_cr
 
-  rts  
-  
+;make and send reply
+  ldax #reply_message
+  stax nb65_param_buffer+NB65_PAYLOAD_POINTER
+
+  ldax #reply_message_length
+  stax nb65_param_buffer+NB65_PAYLOAD_LENGTH
+ 
+  ldax #nb65_param_buffer
+  ldy #NB65_SEND_UDP_PACKET
+  jsr NB65_DISPATCH_VECTOR
+  bcc :+
+  jmp print_errorcode
+:
+  ldax #reply_sent
+  ldy #NB65_PRINT_ASCIIZ
+  jmp NB65_DISPATCH_VECTOR 
+
 do_dns_query: ;AX points at the hostname on entry 
   stax nb65_param_buffer+NB65_DNS_HOSTNAME
 
@@ -309,7 +325,11 @@ recv_from:
 listening:  
   .byte "LISTENING.",13,0
 
-  
+
+reply_sent:  
+  .byte "REPLY SENT.",13,0
+
+
 initialized:  
   .byte " INITIALIZED.",13,0
 
@@ -334,3 +354,8 @@ ok_msg:
  
 dns_lookup_failed_msg:
  .byte "DNS LOOKUP FAILED", 0
+
+reply_message:
+  .byte "PONG!"
+reply_message_end:
+reply_message_length=reply_message_end-reply_message