diff --git a/net/bootp.c b/net/bootp.c
index 3dea70aa..77057c6 100644
--- a/net/bootp.c
+++ b/net/bootp.c
@@ -687,7 +687,7 @@
 	 * Calculate proper packet lengths taking into account the
 	 * variable size of the options field
 	 */
-	pktlen = BOOTP_SIZE - sizeof(bp->bp_vend) + ext_len;
+	pktlen = ((int)(pkt-NetTxPacket)) + BOOTP_HDR_SIZE - sizeof(bp->bp_vend) + ext_len;
 	iplen = BOOTP_HDR_SIZE - sizeof(bp->bp_vend) + ext_len;
 	NetSetIP(iphdr, 0xFFFFFFFFL, PORT_BOOTPS, PORT_BOOTPC, iplen);
 	NetSetTimeout(SELECT_TIMEOUT, BootpTimeout);
@@ -860,7 +860,7 @@
 	NetCopyIP(&OfferedIP, &bp_offer->bp_yiaddr);
 	extlen = DhcpExtended((u8 *)bp->bp_vend, DHCP_REQUEST, NetDHCPServerIP, OfferedIP);
 
-	pktlen = BOOTP_SIZE - sizeof(bp->bp_vend) + extlen;
+	pktlen = ((int)(pkt-NetTxPacket)) + BOOTP_HDR_SIZE - sizeof(bp->bp_vend) + extlen;
 	iplen = BOOTP_HDR_SIZE - sizeof(bp->bp_vend) + extlen;
 	NetSetIP(iphdr, 0xFFFFFFFFL, PORT_BOOTPS, PORT_BOOTPC, iplen);